88 325, no heat at idle (Help please!)

[eight-0-eight]

Hi all,
Wondering if anyone can help me figure this out...I'm stuck.

- 88 325 (SETA) had a leaky heater valve so the previous owner bypassed it when I bought it a few years ago.
- I finally got around to replacing the heater valve, o rings, heater core with brand new parts and un-bypassing it.
- after replacing and bleeding coolant system, I found that I only could get heat if I rev the engine to 2K or higher. Driving around the heat is hot enough, although maybe could be hotter? but once you come to a stop, cold air.
- No overheating so far, temp stays at the half way mark on the gauge during idle. After driving 10-15 around the neighborhood, still is fine. The temp actually drops a little bit when I rev it.

Troubleshooting steps that I took:
- I pulled the hvac controls etc to make sure that everything was correct there from this thread. All fine there.
- Checked the hose orientation was correct (CYL head hose goes to BOTTOM pipe of heater core. Hose coming from back of thermosat, whether spider hose or metal pipe goes to the TOP heater core pipe)
- I've re-checked the heater valve was open and not clogged, everything was clear. When I poured water into the valve, it flowed freely.
- When I turn the heat knob, I can hear the valve working and it does get colder when going to cold, and slightly warmer when going to full hot, when idle.
- Car doesn't seem to overheat. I've had it idle for long periods and have also taken it for 10-15 min drives around my neighborhood
- have attempted to bleed the coolant system of air countless times using different methods (multiple times of each both when engine is cold and warm):

• using attached funnel on ramp method
• replaced thermostat, drilled hole, and used "oil pump" to pump coolant in to top of radiator following this method
• driven around and continued to open bleeder screw for short times to get any air

- timing belt/water pump was replaced under previous ownership a few years ago at a shop but only like 2K miles ago so I didn't want to open that up right now unless I REALLY needed to. I plan to rebuild the engine sometime in the next year, so rather not do things twice, if I don't have to.

I'm okay with it not having hot air during idle from a comfort perspective, but I feel like there is something wrong here and maybe I've overlooked a step and I don't want to find out later when I'm out for a drive.

Does any one have any ideas on how you would attempt to troubleshoot this? Any help would be greatly appreciated!

[82eye]

did you run the heater with the car on ramps while bleeding? if it doesn't get hot air while doing that then you've got an air lock.

[eight-0-eight]

Yeah, I did have the car running with the car up on ramps as well as full heat and full fan speed. Yeah no luck at idle even after letting it get up to temp after 15-20 min...air gets slightly warm but only when revving it to like 2K or higher is when the heat will start to come throughh the vents.

[82eye]

what temp is your thermostat supposed to open? sounds like it opens too early.

edit: may still have some air in the system as well.

[packratbimmer]

I always fill the system through the highest points on the radiator hoses (fill the block in one direction and the radiator in the other) and then fill the reservoir. It works great! Next step is to go through a few warm up cycles with a top off--each time it warms up I crack the bleeder a bit.

[uturn]

In the airbox, there is a flap that directs the air flow across the heater core. Is the temp control knob wire actually moving the flap?

[eight-0-eight]

what temp is your thermostat supposed to open? sounds like it opens too early.
edit: may still have some air in the system as well.

I replaced it with a Mahle 80 °C (176 °F) thermostat and drilled a small hole as was suggested in one of the thread links I posted above. I also cleaned the thermostat housing as there was some mineral build up around the bleed screw area. Yeah I agree on the air and/or some sort of blockage but I just don't know how to proceed. I've drained/refilled the radiator a few times and have also tried to bleed this more times then I can count.

I always fill the system through the highest points on the radiator hoses (fill the block in one direction and the radiator in the other) and then fill the reservoir. It works great! Next step is to go through a few warm up cycles with a top off--each time it warms up I crack the bleeder a bit.

Thank you, do you leave the bleeder screw for extended periods of time during this? I've been just cracking it for a 1-2 sec as there is coolant pouring out but am I supposed to leave it open during the bleed process and keep refilling reservoir?

In the airbox, there is a flap that directs the air flow across the heater core. Is the temp control knob wire actually moving the flap?

Thank you, I did check that as was pointed out in one of the threads I posted above. It does move the flap with ease when moving the temp control.

[82eye]

I replaced it with a Mahle 80 °C (176 °F) thermostat and drilled a small hole as was suggested in one of the thread links I posted above. I also cleaned the thermostat housing as there was some mineral build up around the bleed screw area. Yeah I agree on the air and/or some sort of blockage but I just don't know how to proceed. I've drained/refilled the radiator a few times and have also tried to bleed this more times then I can count.

i honestly wonder if it's working correctly. 80c is standard. it's what i'd go with. where does your temp gauge sit when it's warmed up?

edit: nvm i see you mentioned it sits about where it should.

[eight-0-eight]

Yesterday, I put the front of the car higher on jack stands (instead of ramps) in hopes that the extra height would help any trapped air work it's way out while using the funnel attachment on the reservoir again and both heat and fan speed on highest settings. It didn't immediately help while I let the engine up to temp. A couple small bubbles came out from the funnel and bleeder screw when I cracked it, but barely anything significant. Still same behavior....cold air at idle and warm air when revved. I left it like that overnight in hopes that maybe the air would work it's way out over time. I'll go check it later today to see but I'm not hopeful at this point.

I also forgot to mention that I also used the Harbor Freight coolant vacuum/refill tool the other week and still had the same results.

Is it possible that I have a blockage somewhere? what would be the best way to check that the coolant flow is as it should be? Temporarily replace a piece of hose (maybe the top heater hose at the firewall) with like a clear hose so I can visually see the coolant flowing?

[reelizmpro]

Bottom is the inlet and top is the outlet so it sounds like you have the correct hose orientation. This is typically the culprit. How did the PO bypass the system? There are typically two ways. One way is looping the hoses together and the other is plugging both hoses. Are you certain the hoses aren't plugged at all? I recall my hoses being plugged internally and secured with a clamped bolt on the end. Did you use the new style heater valve or used original?

[packratbimmer]

I always fill the system through the highest points on the radiator hoses (fill the block in one direction and the radiator in the other) and then fill the reservoir. It works great! Next step is to go through a few warm up cycles with a top off--each time it warms up I crack the bleeder a bit.

I forgot to add that I pull a heater hose and fill that part of the system. So to summarize, I fill an empty system from four places. I fill the block, the radiator, the heating system--then seal it up--and follow by topping up the expansion tank filler. I used to have the same problems until I changed my method. It's pretty easy for an air pocket to get stuck in the system and it will not allow flow while just filling at the expansion tank. Ask any E34 folks about how stubborn the 535i is to fill. Bottom line is that using this method, I have turned a potentially several hour burping, bleeding, filling and repeat job into a half hour (and forget about). Just think about it on the blackboard. One is expecting to completely close a system air/fluid tight and then expect flow to occur from one fill point--there is a huge amount of potential for trapped air locked in your cooling system to blow the rest of your afternoon (or longer).

[uturn]

On the chance that the solenoid is old and built up resistance... at idle it's not getting enough juice to stay open. Can you rig up a connector to supply direct power to keep the solenoid open?

Last time someone had an issue with their heat that you are describing, the waterpump blade was spinning on the shaft.

[eight-0-eight]

Jacking it up higher and trying to use funnel to bleed the air didn't work. Still same results. Coolant keeps flowing out of the bleeder valve with no air but still no hot air at idle, only when revving.

Bottom is the inlet and top is the outlet so it sounds like you have the correct hose orientation. This is typically the culprit. How did the PO bypass the system? There are typically two ways. One way is looping the hoses together and the other is plugging both hoses. Are you certain the hoses aren't plugged at all? I recall my hoses being plugged internally and secured with a clamped bolt on the end. Did you use the new style heater valve or used original?

Thank you, they just plugged both hoses. I'll double check that they aren't still plugged. I'm fairly certain they aren't as I can feel the new hoses I added to connect to the aluminum pipes from the firewall were warm when running and I remember coolant coming out from both but it might be worth double checking. I bought this heater valve as it was the direct replacement for the one I had.

I forgot to add that I pull a heater hose and fill that part of the system. So to summarize, I fill an empty system from four places. I fill the block, the radiator, the heating system--then seal it up--and follow by topping up the expansion tank filler. I used to have the same problems until I changed my method. It's pretty easy for an air pocket to get stuck in the system and it will not allow flow while just filling at the expansion tank. Ask any E34 folks about how stubborn the 535i is to fill. Bottom line is that using this method, I have turned a potentially several hour burping, bleeding, filling and repeat job into a half hour (and forget about). Just think about it on the blackboard. One is expecting to completely close a system air/fluid tight and then expect flow to occur from one fill point--there is a huge amount of potential for trapped air locked in your cooling system to blow the rest of your afternoon (or longer).

Thanks, If I can't figure this out, I'll drain again and try to refill using these steps.

On the chance that the solenoid is old and built up resistance... at idle it's not getting enough juice to stay open. Can you rig up a connector to supply direct power to keep the solenoid open?

Last time someone had an issue with their heat that you are describing, the waterpump blade was spinning on the shaft.

Do you mean the solenoid in the heater valve? I wondered that too but think its open by default. I removed it and double checked that the other week with it off and poured water in to check that it was in fact open. I also can hear/feel it working when I change the heater knob from hot to cold. In that case the air also gets colder so I think it's working properly.

I did see someone post in a previous thread about the waterpump being the issue for them. Assuming not, but is there any way of checking this without having to remove the waterpump? I've been trying to avoid replacing waterpump/timing belt right now as it was supposedly replaced ~2K mi ago and was planning to build this as a SETA stroker in the near future, so was planning to wait to replace until then.

[reelizmpro]

I wouldn't trust that URO new style valve. Also, the heater valve on the return pipe is Normally Open. The variable voltage from the temp control knob closes the valve when heat is needed.

[eight-0-eight]

Thank you all for your input and tips on bleeding!

After a few more times of bleeding procedures, and driving further and further with eyes glued to the temp gauge, it finally seems to be getting warmer during idle! I'm not sure if it was air or some sort of blockage but hopefully 🤞 that was it and I'll be good until I eventually rebuild this m20 (and go through this again).

Thank you again!