M20 can't handle the heat (cooling sys ok)

[Vincent Brick]

'91 325i, 225k miles, otherwise runs excellent. Cooling system is legit with oversize aluminum radiator and full service using BMW coolant not long ago, does not overheat or run hot even in extreme conditions.

In temps above 110-115 deg F the car will only handle sustained highway driving for an hour or so. First it will start with a bit of crappy idle and hesitation under load, for a while it will still free rev and get me down the road a ways with very light throttle inputs as long as there are no hills, but will stall if i let off the gas, then eventually any load and it falls on its face and its done. At that point the car will start but then just die, will not idle or free-rev, just a little sputtering and popping, then you have to wait a while for *something* to cool down and then it will run ok again for a little while. Wash, rinse, repeat until temps get below 110 or so, and then the car will go back to running like normal.

This has happened twice now, a year apart, on the same drive to/from my grandfather's house through rural Arizona. I have never had any other similar issue. The one difference this time was that at each point it quit, I was greeted with a bad/old gas smell, like I was rebuilding a motorcycle carb that sat for years. There were no visible fuel leaks or anything, not sure where the smell would be coming from or why. Unplugging the AFM made it worse, so I don't think its IAT-related as I had first speculated.

Fuel pump? That is just about the only major engine service part I have yet to replace on this car. Maybe a dumb question, but vapor lock isn't a thing on these cars, is it? My fuel gauge reads empty at a 1/4 tank so I never really let it run low, and it had plenty in the tank yesterday. Soft lines, injectors and filter replaced within last 2 years.

You don't know fun until you are trying to limp a wounded car 300 miles across a lonely desert with no A/C and black vinyl sport seats in 120+ degree heat and spotty cellular reception. :)

Thanks for any suggestions.

[ForcedFirebird]

Check the round connector under the intake manifold for the injectors, they tend to corrode and make spotty connections. It also houses the coolant temp sensor which the ECU relies on heavily for fueling. They won't run well without an AFM, maybe an idle if you are lucky.

[Vincent Brick]

I cleaned the terminals on that connector with the proper Wurth cleaning tools ~18 mos ago using dielectric grease when reconnecting, and the car has lived in dry environments its whole life, but I will keep that in mind. That temp sensor itself is also only ~18 mos old.

Unfortunately it will be difficult to test any solutions as it is so seldom the car sees such extreme heat.

[ForcedFirebird]

Good. I can't tell you how many times we get a car in the shop that runs poorly and turns out to be that connector.

Since you live in a warm climate (like we do), might want to check the condition of the tank. Seems over the last few years they have been degrading rapidly and often have to change them. I am sure it has to do with the 10% ethanol and it's tendency to attract moisture.

If that checks out, one other issue we had that we chased for weeks was a fuel tank vent was clogged. It would cause the tank pressure to build and would cause fuel starvation, but only when it was hot out, o the car was pushed hard. That is a difficult one to check without pulling your whole vent system apart, so when it happens again, slowly open your fuel cap and see if it is pressurized.

These intermittent gremlins are always the worst :(

[Vincent Brick]

I think you are onto something with the tank, and I am still thinking pump as well for a possibility as well. The car sat for at least 3 years before I got it and I've never cracked the tank open so it is likely due and who knows what funky-ness is going on in there. I just know that old/bad gas smell was strange, like it should be a massive clue to something I'm just not sure what.

I'll try and cop another purge solenoid too to keep in the car in case it happens again and there is indication it is vent-related. I don't recall the vent system consisting of much beyond the lines, canister and solenoid, right?

[ForcedFirebird]

There's also the plastic tank in the rear fender well. I believe the line that was clogged in my previous case was the one from the plastic tank to the fuel tank, but can't quite remember. I just remember when I blew compressed air through the one line, a chunk of booger/rust/fuel/sludge popped out and the car has been great since. In this case, it's a track car, so the solenoid/canister is delete and the vent open to atmosphere.

[ak-]

Was your pump any louder than normal?

[Vincent Brick]

I didn't notice it any louder than normal.

I'll make sure my evap lines are good, but now I think we checked that when the current motor went in.

My mechanic buddy who works on shitloads of E30s at his job still thinks it could be IAT too if not fuel pump and suggested I get another AFM.

I am second guessing whether I redid the fuel soft lines at the filter now. I know I did up front, but I can't specifically remember changing those in the back, I need to confirm, that needs done regardless if I missed it.