Engine dies at idle

BogfordGarage · #1 08-08-2023, 02:13 AM

Hi everyone this one is a bit of a head scratcher but hopefully someone can help who might have experienced a similar problem.

This has happened twice now. Sat at traffic lights after being on a motorway/dual carriageway, foot on the brake waiting for them to change green. All of a sudden it will stall, no warning no stumble or anything. Then it won't restart, cranking for 10 seconds at a time and nothing almost like its run out of fuel.
First instance I turned it over in gear to get out of a live lane on a round about and it restarts, second time I did the same but it didn't restart until I had got out of everyone's way and lifted the bonnet to make sure all was okay, I didn't notice anything out of the ordinary, went to try again and it starts no problem. In both examples it didn't stall again that journey.
My current thinking is it's something electrical causing a loss of 12v to the solenoid. The solenoid itself is actually quite new as I had the old one leaking about a year or two ago and I replaced it with a genuine Bosch one.
Also something to note is I currently have a brake light bulb playing up and since this only happens when I've got my foot on the brake maybe it's something to do with the bulb out circuit.
Hopefully someone can help. Many Thanks in advance
Tristan

ngoma · #2 08-08-2023, 01:45 PM

Is the car in the year range for using the Bosch biodegradable wiring? Mine are, and I have had to repair various intermittent problems with the fuel solenoid, GP circuit, altitude compensator circuit, etc. by running new wire.

You could try a temporary jumper wire from batt+ to solenoid when you are about to go for a drive that includes lots of opportunities for the problem to happen. If no problems, remove the jumper wire, test again without it, and compare.

My other thought was a small air leak into the fuel system, somehow small enough to create problems at idle but not able to affect higher RPM/load conditions.

What is the fuel filter condition?

ngoma · #3 08-08-2023, 01:56 PM

Looks like the biodegradable wiring was 80-87.5, so consider yourself lucky there.

https://turbobricks.com/index.php?th...wiring.317812/

Check carefully the fuse holder for the circuit that energizes the solenoid. Look for excessive heat or melting related evidence.

BogfordGarage · #4 08-09-2023, 03:54 AM

Thanks for your reply Ngoma I will double check the fuse holder. Luckily it's a 940 so doesn't have the biodegradable wiring of the 200 and 700 series.

I may have misdiagnosed the problem however as I now believe, like you suggested, I have a very small leak letting air into the fuel. I was fiddling around doing some voltage checks yes and just thought I should rev the engine while keeping a close eye on the clear fuel line. As it came back to idle a second or two later a large air bubble would travel between the filter and the pump I repeated this a few times and the results were the same but it did not stall. I took the other car to work yesterday but today I've removed the fuel filter and double checked the seals, made sure no old seals weren't stuck on the bracket, topped up the filter and refitted. It started first time no problems runs great but still when revving and coming back to idle I got a large air bubble as before. Any suggestions as to where to look next?
Many thanks
Tristan

BogfordGarage · #5 08-09-2023, 04:03 AM

Fuse holder is all good, no evidence of burns from shorting or blowing. Beginning to think I was wrong about the solenoid supply. I also solved the brake light issue so the bulb out circuit is no longer active under braking.

BogfordGarage · #6 08-09-2023, 04:05 AM

Sorry I forgot to add on about the filter. It's almost new i changed it around 4000 miles before having the problem.

ngoma · #7 08-09-2023, 10:29 AM

If we are suspecting an air ingress leak into the fuel system, a helpful tool is a low-pressure electric fuel pump spliced into the fuel line just upstream of the fuel filter. Watch if the problem clears up. May also expose the leak by dripping fuel out.

Low pressure important because the dynamic timing depends on the pressure differential before/after the IP internal centrifugal lift pump, so don't want to raise pre-pressure too much. Also don't want to run the risk of blowing out some seals.

BogfordGarage · #8 08-10-2023, 03:46 AM

Okay I'll give that a try thanks again Ngoma. What sort of pressure do you think is save? No more than 10psi? Maybe less? Just so I can get an idea of what pump to buy. And do you mean install it before or after the filter?
Also I had a thought earlier, if I disconnect the hose from the fuel tank and also before the filter and plug one end could I pressurise the fuel line with compressed air and try to either hear or see the leak?
Thanks again
Tristan

ngoma · #9 08-10-2023, 09:38 AM

2-5 PSI. Before the filter, to eliminate any possible leak there also.

The reasons for the temporary fuel pump:

The fuel route is under vacuum from the IP centrifugal lift pump section to the tank. Moreso from the IP to through the filter media. By placing the temporary fuel pump ahead of the filter inlet we are slightly pressurizing that section, eliminating the possibility for the air ingress we are suspecting is causing the stalling.

If the stalling problem clears up while using the temporary fuel pump, (I will also be curious to see what happens to the bubble in the clear fuel line section) but returns without it, we can narrow down the cause to air ingress leak and direct our efforts in that direction. Added bonus: the fuel pressurization there may show us the leak location with a "new" or more visible external leak.

Does the stalling happen early on in a journey? Or after an extended hard load high RPM run? Can you see if the bubble is larger when it stalls? Are there any visible fuel leaks at the top, sides, or bottom of the IP?

There shouldn't be any bubble in the clear fuel line section.

I wouldn't subject the fuel system to air compressor pressures. Too much risk of blowing out seals. Unless maybe you are attempting to clear out a suspected clogged hard fuel line between the filter and tank?

BogfordGarage · #10 08-21-2023, 03:01 AM

Hi Ngoma.
I've had a pretty manic week with other cars. I've just got around to properly diagnosing this problem.
In answer to your question it tends to happen after a long time of high RPM, both times it's happened are following highway driving and coming to a stop.
I've not witnessed the air bubble in the stall situation only while trying to diagnose the problem on the driveway while reving the engine. I'm assuming the problem is worsened when the stall occurs and the air causes a lock in either the pump itself or an injection line .
I'm yet to buy an in-line pump but so far I can't find any leaks between the tank and the filter.
Thanks again for your help
Tristan