Fnic99

I would like to ask for the board’s input to address the remaining issue that still affects my car (1991 S4 Auto 85k miles). When I first purchased the car, it ran extremely rich, idle surged, and “Check Engine” light would come on frequently. The problems would only manifest themselves at idle; under load the car ran fine. The computer showed two faults, Temp Sensor and Knock Sensor, both were at the EZK and LH. The service history showed a receipt (2011) from the PO for a recent intake refresh including new knock sensors, hoses, gaskets, and ISV.

My mechanic cleaned the fuel injectors and changed the thermostat which ended the rich condition. The car then passed NJ state emissions inspection. To address the knock sensors, the intake was removed and he discovered that the hose to the ISV was disconnected, one knock sensor was loose, and many brittle or worn rubber parts were not changed during the last refresh. He examined the ISV and determined that it was practically new and moved freely. While he was in there, I told him to also change the CPS.

To address the temp sensor fault, he changed the sensor but the fault still remained. He found that the harness from the temp sensor was frayed and decided to run new wires from the sensor to the EZK and LH. The error still remains for both the LH and EZK (knock sensor gone). The threads of the sensor were also cleaned to ensure proper ground and the LH has been recently rebuilt by a reputable provider.

The problems seem to arise only when the car goes above 170 degrees and primarily when the car is not under load. The idle hunting also seems to come and go. Any ideas on what else could be wrong or the next steps we should take?

Thanks in advance.

John Speake

You seem to have everything covered, but it may be best to double check the temp 2 readings at both LH and EZK when engine is hot. Measured at each 35 way plug with the ECUs unplugged.

You would expect about 200-250 ohms between pins 13 and 17 of the LH and the same reading between pins 19 and 18 EZK.

Pin 17 LH and pin 18 EZK are joined to the ground point MPIX on the rear of the block - check that each in turn is a very low ohms reading to a good chassis ground point.

Fnic99

Thank you for your advice John. I was looking at a diagram of grounds but can't tell if "MPIX" is the ground under the air box. Is this the case?

Thanks again.

MainePorsche

Yes.
MP VIII and MP IX are attached to the bolts that hold the throttle wheel bracket to the block way down under the airbox.

Fnic99

Thanks for the pic. I just took the air box off and the grounds are really buried under there. Is it a difficult job ( for the novice) getting to them?

MainePorsche

Yes, they're deep in there. Easier if the FPR/dampener are off as a unit, as well as removing the MAS. Really easy if intake is off.

Fnic99

Spoke to my mechanic today and gave him a copy of the suggestions. He said he did the exact checks suggested and used an inferred thermal gun to check the sensor temperature. Does anyone have any additional ideas?

Fnic99

Took the air box off and cover and the fuel rail covers It appears that the bolt holding down the ground wire is not fully torqued down. When I push down on the wire with a screwdriver, it moves up and down about 1/2" on the bolt shaft. I would like to tighten it but it appears that you need to remove the FPR and MAF to get to the bolt. I will tell my mechanic. Thanks for the advice and hope this fixes the problem.

Mrmerlin

had an 89GT with this issue,
a loose wire at the V ,
car would sometimes run then cut out,
it would be a good idea to remove both of the bolts,
and clean both sets of ground wires then your car should run perfectly

MainePorsche

Make sure your mechanic knows to route the throttle cable outside the flange, and have whatever fuel line or wire inside the flange. Attached is a pic from a fellow who had it the wrong way.

Fnic99

Thanks for the advice. I would like to attempt to clean the grounds myself (car wrenching neighbor will help). The MAF removal seems to be straight forward; just loosen the collar a little and unplug the connector. Is there any procedure that I should follow before removing the FPR?

MainePorsche

Don't do for at least 10 mins after engine is turned off. The fuel pressure remains elevated for a while even after shut off. You can expect some spillage, but you will have more if you had just turned off the engine. I would take off the fpr, u-shaped line, and dampener as one unit. Basically disconnecting these from the rails. Will give you a lot more clearance. Have a shop rag under each for there will be some spillage of fuel. If you have, or can get, Loctite 574 a thin film applied to the ball/cup joint at the fuel rails will keep leaks away. Also if the MAF is tight getting back in, I have applied a touch of KY or vaseline to facilitate.

Mrmerlin

to drain off the fuel pull the fuel pump fuse and start the car run it till it dies this will remove a good amount of fuel

Imo000

You have KY and Vaseline in stock at home? )

j.kenzie@sbcglobal.net

He's a surgeon. So, yes! I don't keep them in stock, though.
Dave