Mystery...Runs rough when hot..? HELP

notes928 · 12-09-2008, 03:39 AM

Recently bought my 928s Euro 85' and discovered that it sometimes idles poorly and loses power and backfires under accelaration.

Discovered that it only happens when the ambient temperature is hot or when it sits at idle stationary for a decent amount of time (hot or cold weather).

When the weather is cold and it has not been sitting around idling it runs perfectly.

Did the following to work to it:
-New leads, spark plugs, rotors, distributor caps, coil packs, new air filter
-Had the injectors rebuilt
-Cleaned the MAF sensor with CRC MAF sensor cleaner

Can't work out what the problem is!!!!!

Any help would be greatly appreciated!

chesireIsugaree · 12-09-2008, 05:29 AM

Hello,
Did you try to adjust the ignition timing? Are you running premium gas? Fuel filter last done? Run some techron for a couple tanks- do a 100 mile trip on the highway.

These things may help. Could also be the electric fan not kicking on?

jstobo · 12-09-2008, 09:15 AM

I would check for vacuum leaks. Common problem for electronic fuel injected cars. But the hot problem may point to a bad temperature sensor which signals the computer. I do not believe you can set the timing because its computer controlled. (earlier cars you could). But I have an 85 US model so do not know for sure.

Danglerb · 12-09-2008, 11:20 AM

Check the throttle position switch. It is what tells the brain that the throttle is at idle, or WOT, so maybe the top suspect. Make sure the throttle linkage is allowing the full motion as well.

A vacuum line runs from the manifold vacuum either from the plenum or the throttle body back through the firewall to the ignition computer. It needs to go to the correct spots and have no leaks. Check the timing with a timing light as a start.

Is the car in the US, if so it has some kind of "box" added on to federalize the smog, and as they age and go through the hands of many mechanics what they do can become unpredictable.

Sooner or later you need to go through everything on the top of the motor, replace all the vacuum and breather lines, clean the throttle, and check all of the sensors and related wiring.

chesireIsugaree · 12-09-2008, 12:28 PM

sorry I thought this was an older car with adjustable dist.

notes928 · 12-09-2008, 01:10 PM

Thanks for all the replies.

Its a non-us car. I have noticed that the electric fan does not seem to kick in - but the car has not been overheating at all.

Danglerb, what is WOT and what checks can i perform on the Throttle Position Switch.

It only runs bad after a good 10 minutes at idle or in hot wheather so im assuming it has to do with heat soak or hot air going in to the intake.

With vacuum leaks, do they tend to get worse in hot conditions?

What does the Temp sensor do and how can it create rough running?

lizard928s · 12-09-2008, 02:45 PM

It could be the Temp II, O2 sensor (if equiped), or a mis-adjusted/out of calibration MAF.

Danglerb · 12-09-2008, 08:23 PM

Quote:

Originally Posted by notes928

Thanks for all the replies.

Its a non-us car. I have noticed that the electric fan does not seem to kick in - but the car has not been overheating at all.

Danglerb, what is WOT and what checks can i perform on the Throttle Position Switch.

It only runs bad after a good 10 minutes at idle or in hot wheather so im assuming it has to do with heat soak or hot air going in to the intake.

With vacuum leaks, do they tend to get worse in hot conditions?

What does the Temp sensor do and how can it create rough running?

By non US, you mean its never been imported to the US, so its a natural Euro living in Europe?

The mechanical fan should keep it cool, the aux fan in the front is for AC and to prevent overheating if the mechanical fan clutch fails.

WOT is how you drive a Porsche, wide open throttle, accelerator to the floor.

More than one thing happens as it warms up, fuel enrichment gets turned off, and some smog related vacuum circuits become active.

There are two temperature sensors in the front on the top of the coolant bridge, another sensor in the bottom of the air filter box, and the temperature vacuum control I mentioned on the right side head near the MAF. Like the throttle switch all of these need to be checked with a meter, but I don't remember the exact details of how to do it, or what to look for.

************* Maybe skip a bit of that for now.

Let the car warm up, drive it a bit, and allow it to run rough for a few minutes, then shut it off and let it cool down, then pull the spark plugs one at time and look at the color of the insulator near the tip as well as the condition of the tip. Post some pictures if its not a nice light tan color. Dark mean rich, and white means lean, and either means that maybe what you need to start with is the mixture setting adjusted.

notes928 · 12-10-2008, 02:43 AM

The car is an Australian delivery model that lives in Australia. Its the same spec as the euro model cars.

When i replaced the plugs recently they were almost black (not with oil thankfully).

I didn't realise it was possible to adjust the fuel mixture on these cars?

The mechanic checked all the vacuum hoses and replaced the hose for the idle control valve as it was cracked.

I think the workshop manual has info about how to diagnose the temp sensors so ill give it a go.

A lot of what i have read seems to point to the MAF. I emailed the guys at JDS porsche and they seemed pretty certain it was the MAF. Makes sense since the car has done 160,000 miles and as far i know the MAF has not had a service. (Apparently they only last 80,000 kms or so).

Dean_Fuller · 12-10-2008, 09:04 AM

Quote:

Originally Posted by notes928

The car is an Australian delivery model that lives in Australia. Its the same spec as the euro model cars.

When i replaced the plugs recently they were almost black (not with oil thankfully).

I didn't realise it was possible to adjust the fuel mixture on these cars?

The mechanic checked all the vacuum hoses and replaced the hose for the idle control valve as it was cracked.

I think the workshop manual has info about how to diagnose the temp sensors so ill give it a go.

A lot of what i have read seems to point to the MAF. I emailed the guys at JDS porsche and they seemed pretty certain it was the MAF. Makes sense since the car has done 160,000 miles and as far i know the MAF has not had a service. (Apparently they only last 80,000 kms or so).

I would agree the MAF sounds like the problem. See if someone local will swap out theirs just to see....if it fixes the problem then you know...for free.

Danglerb · 12-10-2008, 10:24 AM

JDS really knows what he is talking about, and gives both good advice and service.

If you can get it, CRC MAF cleaner is worth a try, but the MAF wire doesn't last forever.

MAF should have a CO% adjustment via 3mm hex key into a hole on the control box on the side of the MAF. Workshop manual shows the adjustment procedure, and Porken did a write up on how to build a simple test circuit and do the adjustment. Check Porkens site http://www.liftbars.com/home.htm#Blinkr

notes928 · 02-28-2009, 11:28 PM

Thanks for the input and sorry for the delay in posting back results.

Swapped MAF and problem is now fixed. Smooth running and proper acceleration with no stumbles.

Still does not like hot starts for some reason. Starts easy enough but then surges between very low to normal revs before settling down to the correct idle within 5-10 seconds.

Sometimes needs a few cranks before firing when the car or when the ambient temperature is hot.

Danglerb · 03-02-2009, 09:59 AM

Did you ever set the CO%?

Do you have an idle stabilizer valve on your model?

Sertan · 03-02-2009, 11:59 AM

Check the auxiliary air regulator .The auxiliary air regulator heats up and closes off additional air from entering the injection system after the car has warmed up. Rough running when the car is hot may indicate a defective unit

Normy · 03-02-2009, 04:00 PM

Dangler- I have an '85 "Euro" [I prefer the term "S2"] and you can set the CO via a pot on the side of the MAF, but to do it correctly you need a sniffer in the tail-pipe.

[Correctly: EPA emissions correct...that is~]

The manuals detail a procedure where you use a multitester set to ohms to position the pot correctly. 0-1000 ohms; zero is totally rich, 1000 is lean. I have mine set at 900, and my PLX mixture display says that in normal running, I am about 15.0 or so mixture ratio. It varies between 14.0 and about 16.0, so I guess I have it about right. I have a rising-rate fuel pressure regulator [the one from 928 Specialists], and I have it set for a rich mixture. During normal driving, the car runs a bit lean; with full throttle it sits around 12:1, and the performance is perfect.

"Seat of the pants" tuning...

These cars do have an idle stabilizer. It often sticks- pull it off, squirt some WD-40 into it, and put it back in place. The real reason that S2's and other 928's idle funny is either because the Nology plug wires are interfering with the throttle cable...or there are a bunch of vacuum leaks.

N!

Danglerb · 03-02-2009, 07:08 PM

CO% sensor needs to get pre cat gas, via the test tube or with a wideband O2 sensor before the cats.

notes928 · 03-02-2009, 07:46 PM

Haven't set the CO% since swapping out the MAF but will look into.

When the car is hot what would be worse- too lean or too rich mixture?

No vacuum leaks as that has all been checked out. However, the fuel pressure regulators and damper leak a few drops of fuel when i pull off the vacuum hose after running the car. Does this mean they're gone?

Not sure what or where the idle stabiliser is. Will consult the manual about this although the funny idle only lasts a few seconds so i dont think it would be the cause.

Auxilliary air regulator is another item i wil consult the manual about.