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Question Hesitation only the first minutes when the engine is 900rpm on cold startup

Soo..I was wondering if it's normal for a 79 930 turbo to have a stumble issue when you first start it and try to apply gas? If I don't touch the gas the car is fine. It goes through it's warmup process. The stumble only occurs when I try to floor it and let go of the gas or give it more gas without letting it warm up. Now I've seen other 930's drive off as soon as they are started up from cold. Why does mine not like to be driven off as soon as it's started? Could it have something to do with the fuel pump cuttoff switch? Mind you that my 930 is not stock (bigger intercooler, free flow headers, k27). Would and could these mods cause this? Also the WUR is ajusted properly......Also there are no shims in the fuel pressure regulator of the fuel distributor since I've made the WUR adjustable. If the fuel pump cuttoff switch is the problem is there a switch with a certain part number that is better suited for my car???


Last edited by madkow; 06-14-2016 at 11:31 PM..
Old 06-14-2016, 11:20 PM
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Turbofrog
 
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You should invest on AFR meter. It could be too lean for cold opertation even though WUR is dialed in spec. Flooring it when cold should be avoided anyway as the tolerances are big when cold. My car sits pretty rich at cold startup and I can run after I have closed the garage door but I take it very gently with rpms until oil gets some warm in it and WOT only when its close to 180F.
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79 930: K7200R, MS3pro+ID1000+044+E85, Ignition: 123/TUNE/MSD6A, BB+2*turbosmart38, WMI. Weight 1110kg
Old 06-15-2016, 12:29 AM
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Get out your trusty 3mm allen wrench and adjust the idle mixture on the fuel head, just a tad bit richer (clockwise). Also, make sure that both of your fuel pumps are running.
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 06-15-2016, 04:27 AM
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If it runs good when hot and crappy when cold then, generally speaking, the car is rich. When it's cold the car runs rich and as it warms up the WUR leans out the mixture. That's greatly simplified but the logic stands.

More importantly, though, is why the hell are you flooring the gas pedal when you first start your car? You need to be nicer to your car, dude.
Old 06-15-2016, 05:35 AM
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Mighty Meatlocker Turbo
 
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Cow, it's probably not the overboost switch causing the problem, but easy enough to test - remove the wire from it and attach it (the wire) to some nearby metal . . . grounding the wire is what the overboost switch does when it is working right (ground normally interrupted at about 1.2 bar).
Old 06-15-2016, 08:02 AM
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do you still have the AAR / Decel valve? when I ditched mine it got cranky on cold starts and will stumble when you hammer the gas till it warms up.
Old 06-15-2016, 01:45 PM
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Ahhhh....so much conflicting info. What's a person to do? I love this forum, and one thing for sure is that the information is free!

Do everything that everyone has suggested. You'll eventually hit on the truth.
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 06-15-2016, 06:19 PM
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Always start with the basics and work out from there. Does it have full boost when warmed up(intake leak)? If it is fine warm and there are no other symptoms then hesitation on start up is usually a lean or rich problem. Where it is coming from is the question. First thing to always check is the mixture. If you have a wideband, check what the AFR is after a cold startup. A CO meter is better but a wideband will work too. See what it is and report back to us. If it is lean or rich you will want to adjust it and wait until the car is completely cool and see how start up is. If it is still present then you will then want to look at control pressure on the WUR. Is the car stock? If not what does it have. I can tell you for sure it is NOT the overboost switch FWIW.
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Old 06-16-2016, 09:01 AM
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Turbofrog
 
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Why do you say co meter is better? My gunson travels a lot so for me afr is better.
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Old 06-16-2016, 12:03 PM
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Sooooo...I may have most definitly may have found out the problem!

It was a banjo bolt not sealing properly on the top of the wastegate.

The Banjo bolt wrecked the aluminum thread on the top of the wastegate housing which did not have a heli-coil insert. I fixed that. The hose is part of the system that makes the wastegate adjustable with a knob under the dashboard. One hose comes from the Pop off Valve Housing which goes to the knob under the dashboard and from there to the top of the wastegate housing and the other hose comes from the Intercooler and fastens to the side of the wastegate.

I believe that the hose connection with it's close proximity to the Boost Switch and the other end not being sealed properly was the culprit for the stumble on the gas peddle when the engine was cold. The problem was definitly also there when the engine was hot when sometimes on boost it was a little hesitant or moody but rarely.

The RPM's on acceleration are smoother now and since the engine is more air tight now I've had to readjust the idle speed as well as the fuel enrichment.

And now to be German and move on.......
Old 06-16-2016, 10:15 PM
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We call that Dial a Death! Glad you found it. A leak as stated previously would create an issue. Hopefully it is cooler there than here. Makes driving the car miserable.
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Old 06-17-2016, 05:43 AM
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Is there a manual for this dial of death somewhere?

Old 06-19-2016, 04:05 AM
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