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hot restart and running issue

Posted: Jun 21, 2022 12:31 PM
My '88, 535i is having trouble with the Texas summer heat.

Several times now it has repeated the exact same issue:

Driving around town, stop and go traffic, ambient temps of 100+ and the temp gauge is 12:00 or slightly more. I stop the car and on two occasions it slightly backfires and starts, but runs very poorly. At this point I can only give it VERY gradual throttle input or it will stumble (I'm leaning towards fuel delivery because the tach isn't jumping around like it's losing signal). It will continue to drive like this all the way home.

It does not just suddenly do this while driving, only after a restart (even in as short a time as getting gas).

A few days ago, I drove to wash the car. The engine temp was not terribly hot, maybe just 12:00 and certainly not heat-soaked (drove less than 10 minutes). I wash the car (no, I didn't get water under the hood). Go to restart and it simply cranks and no indication of starting, (only once did it give a tiny puff like it fired one cylinder) I popped the hood, pulled on sensors and relays and checked joy. Only after about 20 minutes did it finally crank up and then, again it ran very poorly and I could only give it the most gradual throttle input to gain speed. Step on the throttle too quickly and it acts like it's not getting enough fuel (engine sounds like it's farting...brrrrrrrrrrrt).

I limp it home and pop the hood. Fuel pressure shows maybe down a couple psi from where it normally runs (I've kept a gauge attached in-line now for a couple years). Again pulled/wiggled connections and such with change in running behavior.

Once it cools off completely... it starts instantly and runs perfectly.

In my searching around on here, I read it could be fuel pump relay, in tank pump, or one of the reference sensors.

Any other bright ideas?



Second issue...maybe related? Turn the key and starter will not engage but I hear the relays buzzing. Usually does this when hot, and it gets worse when very hot. Turn the key several times, and when it goes, it engages the starter like normal and the starter spins strong.

Re: hot restart and running issue

Posted: Jun 21, 2022 4:46 PM
by Mike W.
It does sound like a fuel issue. They did sometimes have problems in extreme hot weather, I've experienced it twice, both on long drives, uphill, over 110F, but that's far more extreme than yours. My fuel was so hot I could hear it boiling in the tank. Which is why they put the fuel cooler on that everyone throws away when they do fuel lines.

While I normally don't think the intank pump does much, it might help in your situation. You can either go OE style or do the Vega kludge for cheap.

Re: hot restart and running issue

Posted: Jun 22, 2022 5:41 PM
by tn535i
I've had similar to this happen with a bad CTS. I had one where if it got too hot it had a bad internal connection but when cold it was fine. Just for kicks try unplugging the CTS and see if that is how it feels. I think it goes incredibly lean and behaves like you suggest. You have probably read about this stuff before.

I think it's less likely that you have fuel vapor lock but possibly. Fuel can get so hot it is boiling in the fuel rail and you have too much vapor and not enough liquid fuel. The BMW design is really good about returning excess fuel to the tank rather than dead headed fuel rail and allowing vapor to accumulate. You might also make sure there is no suction being created (remove gas cap).

Re: hot restart and running issue

Posted: Jun 23, 2022 12:27 AM
by Tiit
No use checking fuel pressure while it’s idling. Check the fuel pressure while the symptoms are happening.
These symptoms could be caused by many reasons. I suggest the process of elimination. Check the valve lash, compression, fuel pressure(during the symptoms) AFM and so on. Don’t forget to make a list and tick them off as you go to prevent going around in circles and wasting money in unnecessary parts changes.
Good luck

Re: hot restart and running issue

Posted: Jun 23, 2022 8:21 AM
by Shawn D.
I think it's likely that you're experiencing vapor lock. I agree with Mike that the in-tank pump might be a contributor. Try using non-ethanol gas and see if it mitigates the situation somewhat:

Re: hot restart and running issue

Posted: Jun 23, 2022 5:11 PM
Thanks for the replies everyone.

I drove it yesterday and it ran perfectly for about 15 minutes... then it began this same glitch. I could only apply the slightest throttle or it would stutter. I had to park the car and when I got back in, it would not start. After some 25-30 minutes, it finally fired up, but ran very poorly. I was able to limp it home. Got out and checked sensor connections all over again.

It would not rev up and fuel pressure was normal.

I was damned frustrated and let it sit for a while. I came back to it an hour or so. It started perfectly. It was warmed up and I disconnected the CTS, the idle increased several hundred rpm, but it still ran nicely. I took it for a long drive and could NOT make it glitch...despite it being well over 100 degrees ambient.

If I can make it glitch-out again, I'll disconnect the CTS and see if that helps it...I replaced that CTS about a year ago, by the way.

Stay tuned...