Scotty1

HI Folks , I have an issue with my 1987 s4 starting and idling .It fires up ok but after a few minutes of rough idling like a misfire it dies .also if I press the accelerator there is not much power there .Some of the items recently changed were new plugs ,distrubtor ,ignition ecu ,MAF calibrated ,new fuel pump and filters .the car was sitting for a while so I am not sure if this maybe an injector blocked or possible limp mode .? Before I check anything else out just I wanted to get your thoughts on potential culprit for this issue and to trouble shoot further .I am a newbie to the forum and 928 world so any help or advice would be much appreciated. Thanks all

NickTucker

One thing not mentioned is the ISV (Idle Stabilization Valve) which is unfortunately located under the intake. This valve will, or should, let enough air to pass the throttle plate in order to idle the engine. These will sometimes stick or become slow to react and result in low idle or an erratic one. If you search the forum for "WD40" and "ISV" you should find a quick and dirty trick to try and loosen it up,

A failing LH can also cause rough idle, there is a small daughter board that can crack and cause issues. These are easy enough to get replaced/rebuilt.

Does the engine run ok if you hold your foot on the gas pedal?

dr bob

As Nick suggests in his LH comment, a failing controller will cause the symptoms you shared. If the controller hasn't been reworked already (check for repair/warranty sticker from the rebuilder...) and the car ran to today, it's pretty impressive life. Easiest test is to place your controller in an eligible car (any '87 or later) and see if the symptoms move with the controller.

The "under the intake" stuff includes the idle stabilizer valve (ISV) and associated plumbing. Specifically there are hoses that suffer with age. Even a small backfire in the intake can rupture one of those oil-soaked hoses and cause an air leak. You will ideally test for that massive vacuum leak with a smoke machine, but there are some DIY methods for checking that use some drain pipe fittings and some air from your compressor to add a little test pressure to the intake system. Search for the intake refresh instruction from Dwayne's Garage, which will offer more complete guidance on that ISV and plumbing, plus the details of the test protocol with the compressed air. Related: the oil vent piping from the cam covers and oil filler neck routes to the boot between the MAF and the throttle body, and offers a path for unmetered air that sill cause a lean condition.

The MAF (Mass Airflow Sensor) is critical to the engine operation. The LH controller includes a limp-home function that sets fuel delivery at a fixed rate, one sufficient to get you slowly off a highway to safety but not much more. Low load/low speed. Oddly it isn't engaged with an out-of-spec MAF, but is if you disconnect the MAF completely. Diagnose by unplugging the MAF connector, and see if the engine idles and take a small amount of load/throttle. If so, the MAF is your likely culprit.


Look at a few spark plugs and get more hints at the cause of your symptoms. Dark fuel-rich or even wet plugs are typical to a failing LH, as it dumps fuel into the intake. A lean misfire leaves a black soot on plug electrodes, typical to a vacuum leak from ISV plumbing. "Normal" 928 S4 plug color is a very pale tan, just north of white.

Scotty1

Thanks for your advise hopefully I will get some time this weekend to take a look at these items and try and narrow down the culprit

Scotty1

Just wanted to provide a further update on this issue.the car will not start now at all.it turns over strong and feels like it's about to start but doesn't.i checked for spark and there is strong spark . The plugs were all fouled and were wet and black so I replaced with new plugs still no difference.i then checked all of the relays and fuses and they were all good even jumpered the fuel pump but still no difference.i m thinking that the LH may have failed completely now ? Just wanted to get your thoughts on it before I send to JDS for testing/rebuild or is there anything else I could check .Cam belt looks all good too .thanks

FredR

There are several failure modes for the LH unit- the most common one seems to be when you can hear the injectors clicking but there is no start- this usually suggests the secondary tile has failed and if so the fuel pump wiil not start. You can check for this by inserting a jumper in the fuel pump relay socket across terminals 30 and 87.

Another [less prominent] failure mode is when the car starts - runs for a few minutes and then cuts out out- presumably due to some component heating up and failing.

You did not state where you are located- if you are in the UK you can send the LH unit to Mike in Maidstone and he will test the unit and repair it if necessary. If you are in the States he has agents who can test it for you as per his website http://JDSPorsche.com

Scotty1

HI All, I just wanted to provide a quick update on my troubleshooting and where its at currently .I had the Lh and MAF tested and all checked out with no issues. I cleaned all of the grounds and put new spark plugs and an air filter and new relays and it still would not start .I topped up the fuel thank with about 5 litres of free fuel and tried again and what do you know it fired up idled nice for about 5 min in with some smoke from the tail pipe and then the idle surged and when I pressed the accelerator down it died. I m thinking that this is a fuel related issue the fuel pump and filter is new but perhaps the inside of the tank is dirty or blocking the flow .? Appreciate any ideas on what it might be also would a bad ISV cause the engine to die after warm up ? thanks all

Shark2626

Doubtful that it’s the gasoline because the fuel pump has a large opening to allow for flow and then any junk would be trapped by the filter. And too the car started so your filter doesn’t appear to be clogged.

You said that the plugs were fouled, were they also wet with gas? Check your fuel dampers and fuel pressure regulator to see if they smell of gasoline. You can pull their vacuum lines to smell, you can also gently insert a very small diameter Allen wrench to see if it comes out with gasoline on its tip.

Speedtoys

_where_ are you, might be someone close.

And whats the service history on this car, the fuel system...everything under the hood. You listed a few components, but what about all the hoses, vacuum lines, fuel pressure regulator and dampers...

This is likely death via 100 paper cuts of small issues simply stacked up on top of each other.


"the car was sitting for a while "

HOW long since it last ran well?

GregBBRD

Damper and regulator diaphragm failure is now so common that we check these, before doing any other work on the engine.

The diaphragms crack and break, allowing raw fuel to be injected into the vacuum system...sometimes very significant amounts.

The original Bosch regulators and dampers are now only available from Porsche (and crazy expensive), but I make ecconomical replacements, which can be rebuilt, if the diaphragm or regulator portion fails after years of use.

merope

Does it start if you press the gas pedal?

Scotty1

Thats interesting that you can rebuild the fuel dampers where are you based and what is the typical costs presume its the two dampers at the back of the maf and what about the regulator at the front of the engine that too?

It does start but when you hit the gas it accelerates a little and then dies everything seems to be pointing towards these regulators ?

merope

I think there is some misunderstanding here. He can rebuild his own dampers. The OEM ones can't be opened.

There is a regulator and damper in the back and a damper in the front.

It is not at all certain that this is your issue. But you should check. It solved my idle issues, although they were different from yours. Pull the vacuum lines after the car ran and see if there is any gasoline in them.

Scotty1

Ok thanks for clarifying that it's a replacement damper rather than a rebuild .I started the car for a minute or so and then pulled the vac line off of each damper and they were all dry with no obvious signs of gasoline would this indicate they are good ? Or could there still be a failure here ?.the car does appear to start very rich with alot of smoke and strong smell also

merope

Check the vac line on the regulator also. My "experience" ends here. As I wrote above, I don't think this is your issue, but worth a check.