blue983k

88-928 when the car is cold it will start and run great. just took it out for an hour drive. never turned it off. When I get back home turn it off, let set for 10minutes or even 2 hours go to start it. The car starts and idles normal for 3-4 seconds then the idle starts to bounce then it will kill itself out. Any ideas why the car starts and run s great only one time a day. You can't drive it anywhere and turn it off let it set for any length of time. It will not start and run properly when it is hot. What can it be??

Jack Riffle

If I had to guess, I would say that the check valve in the fuel pump is not holding pressure, although other things can cause this as well, such as a bad Temp 2 sensor. If you had one of Roger's new gauges it would tell that tale in seconds. You might test the Temp 2 sensor with an ohmeter and make sure it's OK.........just my .02

NickT

I had a similar issue with my '88. Drove me nuts for some time. Things to check:

1 - Vacuum Leaks. Biggest pain point here.
2 - ISV valve. But your idling fine at start up (Cold) so I doubt this.
3 - Fuel pressure. Same as above, but worth checking anyway. Pop the vacuum line of each damper/regular and smell for gas. Two under the air box and one at the front.
4 - Normal tune up items.. Plugs wires and so on.
5 - MAF. How old is it and can you swap with a known good one. They have a limited life span.
6 - LH computer. Finally what mine turned out to be. Same symptoms as yours, but I also fixed everything listed above.

Hope that gives you some starting points to check.

NickT

Here are some of my threads on the issue:

https://rennlist.com/forums/showthre...=378734&page=1

https://rennlist.com/forums/928-forum/224378-rough-running.html

https://rennlist.com/forums/928-forum/234425-idle-surge-o2-sensor.html

Again, hope that helps point you in the right direction.

Good luck.

John Speake

Is the car equipped with cats/O2 sensor ?

pdejong

The same problem blew my mind for a while. I had bought a '90GT that had the same problem. It had driven the previous owner crazy. He replaced all hoses, ICV and gaskets under the intake manifold. The car would run perfect when warming up. Here in Houston that goes pretty quick and after a while when the engine would be hot, or when I'd let it sit, like you describe, it would not idle. It would stall every time you slow down for a traffic light, unless I kept my foot on the gas a bit. It wouls also run rough and hunt.
I checked vacuum, sparkplugs etc. No leaks at all. I even took of the intake manifold only to confirm that all was perfect underneath.
Funny enough, the 3 times I messed with the car, I also cleaned out the airfilter. I had noticed bug accumulation every time on a small area right above the MAF inlet. And every time I put things together, the car would be good for a few hundred miles.
To make a long story short this was the problem. The previous owner had the airfilter installed upside down- the arrow pointed into the air flow direction. This put the filter real close to the MAF, within about 1/2 inch. The dirt would accumulate and apparently disturb the airflow in a way that the MAF reacted to it. I installed a K&N filter I happened to have. Older 928's (prior to 83) have a flat airfilter, part # 928.110.185.01. 928's with a MAF (S4's and later) have a filter that looks kind of arched. The high side goes up, which means that the arrows point up. Part# 928.110.185.03. On 928S's (L jetronic), the airfilter housing is much deeper and far away from the airflow meter.
I did not have filters with the word "OBEN" stamped on it, but as it turns out, the arrow means the same and needs to be up and not in the direction of the airflow.
I don't know if all aftermarket filters are marked the same. You may not have the same one. But if your filter is upside down, that may be the culprit.
My car idles better and the improvement in acceleration is noticeable.

Good luck,

Peter, Houston TX
'83 928S AT "europeanized"
'90 GT MT

blue983k

Thanks for all the help. After many hours of testing the relays, changing the air MAF, coil, checking for vaccum leaks, and installing fuel pressure gauge. A great friend Greg S figured it out. It was the oxgen sensor. The PO had spliced in a new one and left the three wires unprotected wrapped in aluminum tape. They were grounding each other out. It was agreat day for 928's when this one was solved.

Thanks again Blue983k

NickT

Glad to hear it all worked out!!!

Enjoy your shark.