Djet cold start issue

[Christopher]

I've long had a cold start issue which would be good to take time to understand, and iron out

My car will start, from cold, on the 3rd of 4th crank and run fine for 4-5 seconds (the cold start opertation working fine). However, the revs will then drop and unless i give it a tap on the gas, the car will stall. All it takes is this one hit on the gas to solve and the car will then idle without further intervention.

The same thing also appears to happen when the car is warmer - it starts fine but then the revs drop, although this time not enough to stall and after a couple more seconds it corrects itself and idles as normal.

The car runs smoothly and without fault after this initial issue. Furthermore all the components were replaced (temp sensors, aav, relays, thermo switch) under my ownership but the issue persists.

Can any more technical, and electrically minded minds point me in any direction to explore for a solution ? Cheers.

 

[Cornishman]

Chris
It sounds like air in the system. So, are all the rubber fuel pipes really good and all clamps suitably tight?
My rubber fuel hoses all deteriorated to the point of failure in about 15 to 20 years and the ones which I replaced when I bought the car went hard in about 12 years. Clearly, I replaced everything again, including those from the fuel rail to injectors. Don’t forget the hoses in the boot, suction and return to tank.
Be warned about mission creep while doing this. Vapour blast C tubes, Rocker Cover, new Water Pump etc.

 

[ScottAndrews]

Chris
It sounds like air in the system. So, are all the rubber fuel pipes really good and all clamps suitably tight?
My rubber fuel hoses all deteriorated to the point of failure in about 15 to 20 years and the ones which I replaced when I bought the car went hard in about 12 years. Clearly, I replaced everything again, including those from the fuel rail to injectors. Don’t forget the hoses in the boot, suction and return to tank.
Be warned about mission creep while doing this. Vapour blast C tubes, Rocker Cover, new Water Pump etc.

Hah Hah!.. Funny you should mention this mission creep.

My euro 635 was having a bad idle at start. I traced it to a leaky valve cover gasket, so, I concur that this is probably a vacuum leak issue. Common sites are: valve cover, oil cap, dip stick, throttle body to plenum boot, and various post throttle body hoses.

And about that scope creep. I removed the air filter, AFM, and AAV to get at the vale cover. But then decided to completely renovate the crusty and rusty air filter housing. After that I decided that it had been 17 years since I had the injectors out, and they have, I believe, never been serviced.

So now I am removing the intake runners, and will send the AFM and injectors out for refurbishment. While that's being done I realized the engine bay is pretty crusty too, and I know the trans seals and clutch need replacing.. so following the example of @Stevehose, the engine is coming out. While that's out I plan to redo the front subframe, paint, bushings, etc..

So yeah, a little vacuum leak can be VERY time consuming and expensive!!

Scott

 

[tochi]

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[Christopher]

Thanks Cornishman & ScottAndrews for the helpful tips.
I've gone though every pinch point this afternoon to make sure everything is tight and all gaskets / orings present - the issue still persists. An air issue would seem to be the most likely culprit, but unless I'm missing something I'm back to square one.

The system runs well at all other times and is only this blip, 3 or 4 seconds into the cold start cycle.

 

[day66]

Might it be worth checking the fuel pressure - perhaps a faulty non-return valve in the fuel pump?

 

[TomHom]

Hi Christopher,

quote you saying
All it takes is this one hit on the gas to solve and the car will then idle without further intervention
unquote

For me it sounds like an issue on the idle position contact of the throttle valve position switch. During the process of starting, the ECU might not get the idle position signal. Only after opening/closing the contacts by moving the gas pedal the signal might be recognised again by the ECU.
The reason could be either bad contact surface or insufficient contact pressure.
Too late in the evening now to elaborate checking
...read my last entry on this threat, a must have if you have D-JETRONIC installed

Thread 'D-Jet testing cart'

Mar 11, 2023

I picked up this antique for $25 on facebook. I’ve never heard of the company and I’m not sure if it works. Missing the “system analyzer” unfortunately Figured the cart was worth that much though.

Keep you posted…

• Markos
• Replies: 9
• Forum: E9 Discussion

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Regards
Thomas