'79 928 with misfiring/stuttering issues

[RogueNine]

Hi there, first post here. I noticed that this forum gets alot of traffic so I figured someone here might have experience with the problem I am having. The other night, I was driving my '79 928 and took a wrong turn down an extremely rough gravel road (the kind that left *my* brain rattled as well) After getting back onto pavement, I noticed that the car had started misfiring and stuttering under power above 3-4K rpm.

Now I have done a bit of testing to find out exactly when this occurs, and it seems that it only happens when the car is warmed up, and under power. It will not misfire when at rest and revving, and will not start misfiring until the car has been running for a good 5-10 minutes. Now, the car was serviced less than 450KM ago, with the fuel pumps being replaced, fuel accumulator replaced, plugs replaced, distributor replaced and a number of other things. The only thing that I opted out of, which was suggested was getting the injectors replaced.

I was wondering if anyone had any experience with this sort of problem, or if you would have any idea what parts would be particularly suceptable to failure under the given conditions. I have a preliminary understanding of the K-Jetronic system, and am lead to believe that either something is wrong with one of the components that adjusts fuel/air at temp, or with the injectors themselves.

Any help would be welcomed and highly appreciated!

Thanks!

[Barbapapa]

Sounds like CIS WUR and/or fuel distributor mischief. The first easy thing to check is the vacuum lines going to the WUR, one of them provides WOT enrichment. Most likely you'll still have problems though. A pressure check will be invaluable. You'll need a CIS gage, either buy one or take it to a shop that has one. There aren't many shops that actually have it but you can always buy a cheap one.
Once you know what the pressures are everything will start to be clear and fall into place. Why not find out and report back?

BTW, the reason it runs better when cold is probably because the cold start valve is working properly, it's independant of everything else though. It's an electric injector controlled by a thermotime switch.

Be prepared that it's still possible to have nothing help until you rebuild the WUR and fuel distributor, just because of the age.

[785sp]

Your best clue is the "extremely rough gravel road" part.

You probably have, on the 79, two fuel filters, one that screws right into the tank and one under your right rear wheel-well. In between these two filters, you probably have two fuel pumps, one that sits under your fuel tank and one that's also under your right rear wheel-well.

Likely, one or more of these four is clogged with whatever you dislodged in your 30 year old fuel tank.

[RogueNine]

Well, after running some 44K through it to no avail, I ordered a fuel filter following 785sp's suggestion. However, after taking the cover off under the RR wheel well, I noticed that the accumulator? is spraying fuel out of the top of it as labelled in the picture.



There is a seemingly capped bolt where the fuel is coming out, would it be safe to lock that down?

Thanks again!

[RogueNine]

I retract that, It seems as if I am more likely in the market for a new fuel accumulator.

[rhjames]

Originally Posted by RogueNine

I retract that, It seems as if I am more likely in the market for a new fuel accumulator.

You have found your problem......


--Russ

[785sp]

Excellent!

By the way, all of my fuel lines in the rear were rotted and leaking when I acquired my beater. I had new ones fabricated at very low cost by taking the old ones to a hydraulic-brake line shop and having new rubber crimped onto the old hard parts. There should be one somewhere in your area, for the trucking trade.

The trick is tightening down the "banjo" bolts sufficiently when you re-assemble. There are various posts in here about that, and you might need to buy some new wrenches and/or washers. I used vice-grips on a couple.

[ROG100]

be carefull on the accumulator as there is a VIN split for MY79.
928 110 197 05 or 928 110 197 07

I stock both and the 05 is $169.95 & the 07 is $149.95

Let me know if I can help you further Roger@************

[rhjames]

Originally Posted by 785sp

Excellent!

By the way, all of my fuel lines in the rear were rotted and leaking when I acquired my beater. I had new ones fabricated at very low cost by taking the old ones to a hydraulic-brake line shop and having new rubber crimped onto the old hard parts. There should be one somewhere in your area, for the trucking trade.

Your local Caterpillar dealer can help with hydraulics lines.

anything tough enough for a dozer is tough enough for a Porsche (ha, ha, ha.....)


--Russ