Richard Andrews

Hello, This is my first post on Rennlist, and I am an admirer of all the knowledgeable members that provide their technical input, advice, and guidance to other members. I am preparing to sell my 1988 S4 on Bring a Trailer in a couple of weeks, and I have only one remaining issue with the car that has appeared in the last month. The car has been completely restored with over $25k invested in parts alone, and every system in the car (except the engine bottom end) has been rebuilt or refurbished. It is my goal to state honestly that the car is flawless and immaculate both cosmetically and mechanically, but I can't do so unless I get this last irritating problem resolved - hopefully due to a members' wise counsel. My problem is:When the car is cold or has sat overnight it starts on the first crank, and idles and runs great. Even when it is at engine temperature, when I am at a stop light, it idles perfectly and runs perfectly. The issue happens when I stop and turn the car off with the engine warm and then try to restart the car. It starts right away in park (it is an automatic), but will stumble and stall when I put it in gear. It will do this until I give it some gas to keep it running and drive away. If I stop within a hundred yards or less it will not idle and will stumble and die if it is in gear. If I drive a mile or so and stop, it acts normally and runs great until I stop again and turn it off and try to restart it. It is frustrating and obviously something that I need to resolve before the auction. Once the car cools down its back to perfect.

Everything related to the fuel delivery system is new, but now about ten years old – but with less than 500 miles. (It was a slow ten year restoration!). I spent all day yesterday finishing the tune up with the new plugs, re-tensioning the belt, oil change etc., and the tank is half full with good 93 octane non-ethanol gas. Other than this issue it is running great. I also checked all connections for vacuum leaks, etc., and cleaned all the ground points and checked all the fuses and relays.

Responses are greatly appreciated!

Richard

Geo55

Have you checked whether or not it holds fuel pressure at the fuel rail. you may have a problem with some injectors sticking open. first I though of damper, but if they are new....

GregBBRD

1. Raw fuel leaking into vacuum system from fuel pressure regulator or damper?
2. A bad MAF? (The O2 sensor ultimately controls the fuel mixture, however it takes a couple of minutes of running for it to get hot enough to take over and correct what information the MAF is sending to the LH control unit.)

Richard Andrews

Thanks for the suggestions. Could there be a contributing problem back at the tank with fuel pump, filter, etc.? This thing has me stumped because otherwise the car runs great with absolutely no other issues.

GregBBRD

Can't think of anything, in the rear of your car, that would cause your described problem.

jchasty

Hi

It's a misleading thread title, but the underlying issue sounds very similar to you!

Jerky Gear change - with a thunk! - Rennlist - Porsche Discussion Forums

Hope that helps

John

IcemanG17

My first thoughts are a sticky idle valve. Its an easy test, just search for it remove the hose and spray some WD40 down the proper hose....if it smooths out....you found it. My 88 S4 auto did this and a new idle valve fixed it......over time the oil injected into the intake makes them "sticky" and the WD40 briefly cleans them.

Popoboy944

To me, it's sounds like you may have what some call a "hot fuel handling" or "vapor lock" problem. Basically on hot restarts the fuel in the rails/underhood fuel system has been heat soaked. If the temperature of the fuel exceeds the boiling point for the pressure it is being held at, it will become part vapor. Then when the injectors go to inject liquid fuel, they get a mix of vapor/liquid, which is much less than the expected amount of fuel. The issue will happen on hot restarts and persist until the hot fuel can either be burned or returned to the tank. Then liquid fuel enters the rails and all is fine again. Considering the car runs so well the rest of the time, and when hot but before shutting off, odds are high you have vapor lock. I am quite familiar with the engineering behind this, and have solved this issue on one of our race cars, but I have never specifically trouble shot a 928 for this. Two generic things that help. #1, keep the fuel as cool as possible as it makes it's way to the engine. Double check the routing of all the fuel lines checking if they are near things that are very hot and shouldn't be, and if any heat shields are required. I'm guessing it's probably not this as I don't think the fuel lines on a 928 can be that out of place. #2, keep the pressure up! There should be a check valve system somewhere in the engine bay to hold fuel pressure when the car is off. It is there in part exactly for this issue. Is the pressure is leaking out of the rails, you'll have this issue. Probably your most likely culprit in this case. I imagine there is a lot of info on the forum explaining that part of the fuel system. Best of luck!

jchasty

I have the same issue - see the video below (it's safe)https://1drv.ms/v/s!AiccFb8_jADak4k_QEGLLbFwW9PieQ

ISV, TPS etc. all replaced.....driving me crazy!!

jchasty

Hope this is useful, I'm 99% sure we've found the issue to poor warm idling.

I'm sure everyone knows this, but new to me: there's a coding plug that tells the LH what type of engine is installed, cats fitted, region etc. This has a big effect on the O2 mixture.

More tech speak below, but you can see the problem in the pics...! A corroded pin - now cleaned up - fingers crossed!!!




Credit to Nick, the classic tech at Porsche Centre Bournemouth for finding the issue, and Michael at JDSPorsche for the following info:

"From your coding plug you have a M249 + cats. Pins 4 and 8 are to tell the LH to use 02 correction because you have cats. I imagine if there's no connection the LH will not account for it.

Pin 1 is ground, 2 and 7 are coding for the LH and EZK respectively.

The pins to the ECUs are collectively called 'oxygen family' in the technical literature. The earlier S4 before the 02 sensor has an adjustment pot for idle mixture near the fuse panel, the coding plug was very similar.

In basic terms the ECU gets air mass in from the MAF and oxygen out from the sensor in the exhaust and is called closed loop. If one of those sensors goes wrong the ECU will get the wrong information and the mixture as a result will be wrong. In the event that one of them fails completely closed or open circuit the ECU will default to a 'limp mode' fixed injector pulse."

Hope this helps the OP!

John

worf928

What Greg wrote -^ swapped in priority.

Mass-Air Sensor

Unless you had it rebuilt during your 10-year long restoration, it’s done. Pull it and send it out for testing.

When an auto-box stalls coming to a stop when warm… usually the MAS.

blwnstrkd

Is there a coding connector available to purchase for a 1988 928 S4 auto trans