'84 ran poorly this afternoon during a weekly start-up

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04 Dec 2011 18:26 #7271 by Kiln_Red
As most of you guys may know by now, I am repainting my car and making very slow progress on the body and paint work. Nonetheless, it has been progress. Anyway, I have obviously not been driving the 928 for a couple of months now. I have been starting it on a weekly basis, though, and allowing it to warm up and idle.

This afternoon was different. I started the car and immediately noticed a faster than normal and rougher idle. Moments later, I noticed the catalytic converter was smoking a little as it was burning off raw fuel. So, it was obviously running rich. I searched for vacuum leaks as this was my first suspicion. I only found one obvious leak. The metal vac line that runs to the front of the intake manifold was leaking where it is supposed to meet the rubber vac hose that continues all the way to the vac modulator at the transmission. The rubber line was oddly totally missing. I don't know when or how it would have came up missing. The car was shifting smoothly and on time the last time I drove it.

It is obviously going to be quite a pain to replace the vac line. I don't look forward to it. I wanted to ask you guys if you felt that the metal vac line would be enough of a source for a vac leak to allow the car's idling and fuel mixture to change that much? It seems odd to me because the exhaust was actually producing black smoke. It was running that rich. What do you all think? :confused:

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04 Dec 2011 21:24 #7273 by 928mac
I think it is either your fuel pressure reg or the dampener that is sucking fuel through the vac line. pull the vac lines on them and check for fuel.

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04 Dec 2011 21:24 #7274 by pcar928fan
I rather doubt it... You can have quite a few vac leaks on these cars and they still run fairly well. Sounds like it was running in 4 cyl limp mode. I'd disconnect the battery for 5 or 10 min and hook it back up and see what happens...

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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04 Dec 2011 22:25 #7275 by Kiln_Red
I have got a feeling that this is going to be fun to diagnose. James, I'm with you. It seems doubtful that one single vac leak would make such an impact on the fuel/air mix right at start-up when it ran really well during the last run cycle.

I haven't had any fuel issues on this 928 before and don't have a great wealth of knowledge on L-jet for that reason. I guess I need to get educated. In the mean time, maybe clean the grounds and see if that impacts it any. I put a new o2 sensor on the car about 6 weeks ago. Maybe that's worth mentioning. I'm already confused and I haven't even got started yet. It's very frustrating as I'm right in the middle of working on cosmetic related things on this car. :mad:

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10 Dec 2011 16:26 #7339 by Kiln_Red
I have an instinct feeling that my temp time sensor has gone south. Usually, when these fail, as most of you know, the car quits starting in cool ambient temps. I think mine has failed in a much more rare way whereas the sensor never shuts the cold start injector off. I will test in a few and follow up. I am ready to continue the progress I was making with the body and paint work. :D

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10 Dec 2011 16:40 #7340 by 928mac
Sounds very reasonable Austin

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