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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > DIY Guides > DIY: Flushing the E9x Heater Core (Caution Required!)



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      11-18-2024, 11:22 PM   #23
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Originally Posted by leif20 View Post
You know I was just thinking that. Driver's side heat is ok but passenger is tepid. I am sure there are still blockages in the core. I would expect no temperature deltas if the core was operating correctly. I did NOT fully pressurize but I did use maybe 1/3rd the pressure of the garden hose. I was anxious and worried to blow out the heater core and render my car undrivable. Garden hose pressure is 40-80psi? I am worried this will burst the heater, given typical coolant system pressures are 15psi ish.

I might speed up the process and repeat. Given i've already done it once, I don't think I need to run vinegar again for 2 hours. Maybe CLR for an hour. How much should I open the hose? I do have a regulator at the barb so i can carefully control the flow there.
Yes, a valid concern. Household water pressure is 40-80psi. That's why you never want to expose a blocked core to instant pressure from a hose.

However, it is important to note that the water pressure inside an open hose, flowing fully with no restriction at the end, will be almost 0. Only if you block the end of the hose, will the pressure inside increase rapidly to the max.

That's why it's recommended to first dissolve the gunk that binds any sediment, then use a nozzle with flow control to gradually increase flow until the core is clear of sediment and able to handle the full flow from the hose.

To prevent accidental over-pressurization, I pulled the pump tubing out of the bottom connection of the core, leaving a tube only in the top of the core. Then I made a fist around the loose end of that tube and the nozzle of the garden hose. Any sudden over-pressure situation will burst out of your fingers rather than the core. A handful of sediment popped out of the core and a second later, the core was flowing a fully open nozzle with hardly any pressure felt in my fist.

Like this... Leave a gap between tube and nozzle, then squeeze hand shut to seal...

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Last edited by GSB; 11-19-2024 at 12:29 AM..
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      11-19-2024, 11:06 AM   #24
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Originally Posted by GSB View Post
Nice! Thanks for sharing. I'm glad it went well for you. Cool idea with the filter. During your step #4, did you also pressurize gradually with a garden hose? I found that some of the loosened particulates were so heavy that the low-pressure pump alone could not blast them out.
I did but didn't push it too hard as I was terrified of blowing a seal or causing a leak. Just a couple short bursts with the hose at the end of the process and not at 100% water pressure. I actually think the CLR was dissolving some of the bigger flakes. If I got something big enough in the filter to pick out with my fingers it would just sort of disintegrate when I touched it.
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      11-19-2024, 11:34 AM   #25
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Looks like i have to repeat the job. Hopefully will go quicker this time. I'll use the superclean and CLR, and then the hand-regulated garden hose. Brilliant idea there!

I guess simple green and vinegar weren't strong enough.

sepling did you use any hot water in the process?
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      11-19-2024, 11:52 AM   #26
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Originally Posted by leif20 View Post
Looks like i have to repeat the job. Hopefully will go quicker this time. I'll use the superclean and CLR, and then the hand-regulated garden hose. Brilliant idea there!

I guess simple green and vinegar weren't strong enough.

sepling did you use any hot water in the process?
Yes. Both my degreaser and CLR flushes were hot water though it cools down pretty quickly when circulating the solution. I ran the CLR for probably two hours switching the flow direction every thirty minutes or so. As long as you aren't leaking where the vinyl tubes enter the heater hoses, I see no reason why you couldn't run the CLR for much longer, but only if it's a last ditch effort as it is etching the aluminum.

Last edited by sepling; 11-19-2024 at 11:59 AM..
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      11-19-2024, 12:14 PM   #27
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Originally Posted by sepling View Post
Yes. Both my degreaser and CLR flushes were hot water though it cools down pretty quickly when circulating the solution. I ran the CLR for probably two hours switching the flow direction every thirty minutes or so. As long as you aren't leaking where the vinyl tubes enter the heater hoses, I see no reason why you couldn't run the CLR for much longer, but only if it's a last ditch effort as it is etching the aluminum.
Interesting. CLR is NOT recommended for aluminum, I guess the idea is to run it only for as long as necessary? I do get some leaks from the vinyl hoses but I didn't use masking tape. Maybe I will do that first.

How did you heat up the CLR? Mix with hot water?

GSB After all the chemical treatments, Are you suggesting to do the pressurized back-flush first, meaning running it backwards through the core to blow out the sediment? Then once it's flowing more freely, flush in the forward direction as well? Intuitively this makes sense to me, back-flush with pressure to remove the blockages.

Thank you both for all the help! If this doesnt work the next time I'm going to start thinking about replacing the heater core.
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      11-19-2024, 05:43 PM   #28
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Quote:
Originally Posted by leif20 View Post
Interesting. CLR is NOT recommended for aluminum, I guess the idea is to run it only for as long as necessary? I do get some leaks from the vinyl hoses but I didn't use masking tape. Maybe I will do that first.

How did you heat up the CLR? Mix with hot water?

GSB After all the chemical treatments, Are you suggesting to do the pressurized back-flush first, meaning running it backwards through the core to blow out the sediment? Then once it's flowing more freely, flush in the forward direction as well? Intuitively this makes sense to me, back-flush with pressure to remove the blockages.

Thank you both for all the help! If this doesnt work the next time I'm going to start thinking about replacing the heater core.
If CLR is not recommended for aluminum, you probably shouldn't be using it. Vinegar, degreaser and dishwashing detergent are the safer, tried and tested methods.

Remember that a chemical flush is not going to dissolve the sediment. It will only dissolve the brown/grey sludge that binds the sediment to the tiny tubes inside the core. Much of the sediment consists of heavier metallic or sand-like particles that sink into the bottom of the core. Judicious pressurization of the core afterwards is required to blow those particles out. Pressurizing from the top of the core ensures that those sunken particles will exit the bottom of the core more easily, rather than forcing them upwards through the tiny tubes in the core. Once the core is flowing freely, reverse the flow to blow out any particles that may have become stuck.

As mentioned previously, before pressurizing from the top, remove your tubes/hoses from the lower outlet of the core to prevent any kind of restriction to the outgoing flow!

Last edited by GSB; 11-20-2024 at 01:52 AM..
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      11-23-2024, 01:16 PM   #29
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I've just done this again. I ran the full pressure of my garden hose though the core, no blockages in either direction. Mind you I was able to plug my garden hose by covering it with my thumb, so maybe my house has some sort of pressure relief in the system (leaky pipes?). So I am not sure if i was really giving it 40psi, or if there's a small blockage that's deep on the passenger side and really stuck there. It appeared as if there was full flow through the HC. This DIY did help me tremendously, so thanks for that. One tip that is not mentioned, get a very looooong needlenose pliers for the heater core outlet host clamp. Took me 1 minute to get it off this time vs. having to struggle with standard length pliers. Also, both times I did the HC inlet, the little retaining clip popped off and fell on the ground.

However, my over-cooling still persists. Fan comes on around 70C and cycles on/off, keeping the temp at 70 rather than letting the car get real hot. I'm thinking to replace the ECT and radiator out sensors, but i am now in the 'throwing parts' stage of a problem. I wonder if there is a blockage in one of the hoses somewhere, because it doesn't seem to be in the heater core.
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      11-24-2024, 05:38 PM   #30
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Quote:
Originally Posted by leif20 View Post
I've just done this again. I ran the full pressure of my garden hose though the core, no blockages in either direction. Mind you I was able to plug my garden hose by covering it with my thumb, so maybe my house has some sort of pressure relief in the system (leaky pipes?). So I am not sure if i was really giving it 40psi, or if there's a small blockage that's deep on the passenger side and really stuck there. It appeared as if there was full flow through the HC. This DIY did help me tremendously, so thanks for that. One tip that is not mentioned, get a very looooong needlenose pliers for the heater core outlet host clamp. Took me 1 minute to get it off this time vs. having to struggle with standard length pliers. Also, both times I did the HC inlet, the little retaining clip popped off and fell on the ground.

However, my over-cooling still persists. Fan comes on around 70C and cycles on/off, keeping the temp at 70 rather than letting the car get real hot. I'm thinking to replace the ECT and radiator out sensors, but i am now in the 'throwing parts' stage of a problem. I wonder if there is a blockage in one of the hoses somewhere, because it doesn't seem to be in the heater core.
Dang! The aggravation of car repairs! Yes, there is certainly something else wrong if the fan is kicking on and off at 70 degrees. A blockage, a bad sensor, a malfunction of the thermostat/waterpump are all possibilities. Check your error codes for clues.

If you suspect blockages, you can run dishwasher detergent throughout the entire cooling system before flushing and replacing the coolant.
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      12-02-2024, 08:04 AM   #31
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Update on my situation, I think it was a combination of the blocked heater core and faulty thermostat causing the temps to drop. I did have a P1028 and I do think the thermostat was opening too soon. I replaced with a 'URO Parts' one (cheap), but when taking the car out on the highway I still couldnt get operating temps. Both ECT and Radiator Outlet sensors had correct resistance readings (5k ohms at 5deg C, increasing linearly reading 250 ohms at 80C). All this trouble started after my mechanic replaced the radiator about 2y ago. Also, thermostats have a bypass hole in them (some withe a one-way valve) to protect against situations where the thermostat might fail closed. This bypass allows some radiator coolant to flow in to the closed loop, and in my case it seemed like the radiator coolant was dramatically cooling off the remaining circuit.

After doing a bunch of driving, monitoring water pump speed, fan speed, and temps in INPA, I realized that the new rad might be a bit too powerful for the cooling system and whatever issues it has as a result of being 19 years old and 289,000km worn. When activating the blower fan with HVAC set to heat, the water pump spins up to 76% and stays there, as a baseline, to keep circulating hot coolant. It will spin up higher if cooling demand needs it, but on my car 76% is the minimum when the heat is on. With the faster pump speed, the cold rad coolant bypassing in to the smaller warm up made me lose a lot of engine heat. The problem would be worse once the thermostat opened. I do think I had some blockages, and still have some blockages in the heater core. This procedure helped for sure, A lot of crap came out when I did it, but unfortunately I wasn't able to get everything out.

My solution? Cover up the front grilles with duct tape. I'll be replacing it with cardboard and then possibly something 3d-printed. But for now, I am building heat and retaining it. Highway driving used to be a freezing experience but once the car is warm, it stays warm. I am sure i have some other issues with the system, but for now, in colder weather, this is my solution.

Using Bimmerlink with DCAN cable to monitor ECT while driving as well, to avoid overheating.
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Last edited by leif20; 12-02-2024 at 08:12 AM..
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      12-03-2024, 01:05 AM   #32
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After doing a bunch of driving, monitoring water pump speed, fan speed, and temps in INPA, I realized that the new rad might be a bit too powerful for the cooling system and whatever issues it has as a result of being 19 years old and 289,000km worn.
Wow. Crazy! What kind of radiator did your mechanic install?

It is highly unlikely that a blocked heater core, or any kind of blockage, will result in overcooling. That is far more likely to cause overheating. While an over-rated radiator could be to blame, my first suspicion would be a malfunctioning thermostat - stuck open. The electrical thermostat in my car has only two wires to power the solenoid. There is no open/closed feedback for the DME. The DME only knows whether thermostat voltage has been activated or not. If the valve jams open, the DME is not going to know, and the engine will overcool. However, since you replaced the thermostat and saw no change, I wonder if the thermostat voltage is actually making it all the way to the thermostat? You might try sticking a voltmeter into the connector or wiring harness right at the thermostat.

Last edited by GSB; 12-03-2024 at 01:15 AM..
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      02-05-2025, 08:43 AM   #33
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Nice write up OP. Very well done. On my to do list asap. Ty, Ric
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      02-05-2025, 03:47 PM   #34
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Nice write up OP. Very well done. On my to do list asap. Ty, Ric
You're welcome. I hope it works out well for you. One year later, my core is still working perfectly, while outside temperatures have been below freezing for weeks.
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      03-05-2025, 11:11 AM   #35
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This is a great thread, thanks to you all. I am soon to embark on this process having eliminated IHKA, thermostat, flaps functionality. I have same problem, cold on drivers side and warmish on passenger side. Before I start I just want to be sure which is the inlet and outlet on the firewall to the matrix. The way I am reading it is TOP=Matrix outlet, BOTTOM = Matrix inlet. Hope I am correct. I plan to try inserting a much smaller ID pipe through both inlet and outlet to try and disturb any blockage at the bottom of the matrix. Any thoughts welcome.
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      03-07-2025, 02:41 AM   #36
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This is a great thread, thanks to you all. I am soon to embark on this process having eliminated IHKA, thermostat, flaps functionality. I have same problem, cold on drivers side and warmish on passenger side. Before I start I just want to be sure which is the inlet and outlet on the firewall to the matrix. The way I am reading it is TOP=Matrix outlet, BOTTOM = Matrix inlet. Hope I am correct. I plan to try inserting a much smaller ID pipe through both inlet and outlet to try and disturb any blockage at the bottom of the matrix. Any thoughts welcome.
That is correct. TOP=Heater core outlet, BOTTOM = Heater core inlet.

However, after the alkali/acid clean, when blasting out the core, you want to reverse the flow first, gradually pressurizing the TOP of the core and pumping the sediment out of the BOTTOM of the core. Otherwise, you'll be forcing all of that heavy sediment UPWARDS through the core cooling tubes, potentially blocking them and bursting them. Once cleared out, the flow should be strong, able to support the full flow of a garden hose. Only then, pressurize the BOTTOM of the core to flow the opposite direction.

Last edited by GSB; 03-07-2025 at 02:47 AM..
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      03-07-2025, 10:06 AM   #37
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My preferred method is just to spring a coolant leak about every 3 months so I always have to put fresh coolant in. You should see how clear it is--absolutely amazing for a 16 year old car
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      03-07-2025, 03:58 PM   #38
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Hi everyone, today has been a good day. I finally took on the flushing job. I bought two 3/4” PVC hoses 2m long to attach to inlet and outlet. Then set up a pedal bin with a pond pump at the bottom with inlet filter. One pipe attached to pump discharge and the other pipe dangling in the fluid. I added Irontite Thoroflush to the fluid, expensive stuff but seemed to have good reviews. I also made up a pipe attached to mains water using plant irrigation system, 5mm diameter pipe. I cut the end to about 20 degrees (like a mastic gun) and formed a point. This meant the water discharge could go in all sorts of directions when the point hit an edge and bent over.
I connected up the two ¾” pipes first and did alternating forward and reverse flushes for about ten minutes each. In between each flush I removed the bigger hoses and inserted the 5mm pipe into the bulkhead connections and carefully poked down as far as it would go alternating between inlet and outlet.
I did this routine for about an hour, put bulkhead pipes back and off for a test drive. Once engine temp came up heat was coming from all vents. When I got back home I checked the centre vent temps and was getting 45degC left and 41 degC right. Previously it was 28degC and 20 degC. Amazing result, maybe not perfect but I have a warm car now.
A BMW specialist garage gave me an estimate to replace the matrix of £1500-£1800. The pipe cost me £12, everything else I already had.
I have no idea if the success was down to the Irontite powder, the 5mm gadget or just normal flushing but hey, it worked. I will monitor then vent temps to see if the fix lasts but happy knowing it is pretty easy to do this flush again if some more stuff blocks the matrix. Hope this helps someone in the future.
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      03-08-2025, 12:44 AM   #39
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Quote:
Originally Posted by Gavin335DBMWOwner View Post
Hi everyone, today has been a good day. I finally took on the flushing job. I bought two 3/4” PVC hoses 2m long to attach to inlet and outlet. Then set up a pedal bin with a pond pump at the bottom with inlet filter. One pipe attached to pump discharge and the other pipe dangling in the fluid. I added Irontite Thoroflush to the fluid, expensive stuff but seemed to have good reviews. I also made up a pipe attached to mains water using plant irrigation system, 5mm diameter pipe. I cut the end to about 20 degrees (like a mastic gun) and formed a point. This meant the water discharge could go in all sorts of directions when the point hit an edge and bent over.
I connected up the two ¾” pipes first and did alternating forward and reverse flushes for about ten minutes each. In between each flush I removed the bigger hoses and inserted the 5mm pipe into the bulkhead connections and carefully poked down as far as it would go alternating between inlet and outlet.
I did this routine for about an hour, put bulkhead pipes back and off for a test drive. Once engine temp came up heat was coming from all vents. When I got back home I checked the centre vent temps and was getting 45degC left and 41 degC right. Previously it was 28degC and 20 degC. Amazing result, maybe not perfect but I have a warm car now.
A BMW specialist garage gave me an estimate to replace the matrix of £1500-£1800. The pipe cost me £12, everything else I already had.
I have no idea if the success was down to the Irontite powder, the 5mm gadget or just normal flushing but hey, it worked. I will monitor then vent temps to see if the fix lasts but happy knowing it is pretty easy to do this flush again if some more stuff blocks the matrix. Hope this helps someone in the future.
Nice to hear that you had a good result, even doubling the heater temperature, with little money spent.

Hopefully, when you connected up the two ¾” pipes and did alternating forward and reverse flushes, you were using the full volume of the garden hose and not the limited volume from a 5mm drip irrigation spigot. That would be important to maximize the ejection of heavy sediment that has to travel uphill to come out of the core.
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      03-08-2025, 12:54 AM   #40
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My preferred method is just to spring a coolant leak about every 3 months so I always have to put fresh coolant in. You should see how clear it is--absolutely amazing for a 16 year old car
LOL! A very logical theory. However, I'm sorry to report that this doesn't keep the heater core clear.

I cleared my heater core only a few months ago and replaced the coolant. Last weekend, I replaced a blown radiator for the third time, and I took the opportunity to blast out the heater core again. Sure enough, MORE sediment was collecting in the core and reducing flow. The coolant looked perfectly clean, only because the heater core is behaving like a coolant filter and trapping all of the particles in the system!
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      03-10-2025, 08:45 AM   #41
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LOL! A very logical theory. However, I'm sorry to report that this doesn't keep the heater core clear.

I cleared my heater core only a few months ago and replaced the coolant. Last weekend, I replaced a blown radiator for the third time, and I took the opportunity to blast out the heater core again. Sure enough, MORE sediment was collecting in the core and reducing flow. The coolant looked perfectly clean, only because the heater core is behaving like a coolant filter and trapping all of the particles in the system!
Dang, I didn't know that. I thought that only happened if people don't change their coolant in eons or don't use the heater often.
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      03-10-2025, 01:59 PM   #42
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Dang, I didn't know that. I thought that only happened if people don't change their coolant in eons or don't use the heater often.
Unfortunately not. In this engine, the coolant is circulating through the heater-core full-time, from the moment the ignition activates the water pump.

The sediment comes from the gradual breakdown of temperature-cycled metals and other materials in the engine block, radiator, pump, and so on.

These sediments are heavier than the coolant, so they precipitate out of the coolant, sinking into crevices and becoming trapped in the fine tubes in the heater core and elsewhere. Pressure is the only way to blast them out. However, any sludge that may bind the sediment must be dissolved first.
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