Advice sought on rough running
 

My 1987 928 S4 has the following symptoms:

• Sometimes takes extra cranking to start
• When it starts, it often sputters and is on the verge of dying, sounding like it's not running on all cylinders
• Sometimes it idles fine and I can rev it just fine and it runs like a top for a short period (5 seconds to a half a minute)
• The symptoms came on gradually. Initially, just acting like it was occasionally missing while at a stop light. Then, acting like it was missing a little more regularly under acceleration. The symptoms went from zero to full on sputtering and dying in about 30 miles, or 45 minutes, of driving.
• The symptoms did start shortly after a fill up

The relevant work that I did before the problems:

• New timing belt
• New cams
• New oil pump gear
• New crankshaft gear
• New water pump
• New Porkensioner

After that work, I put a few hundred miles on it with zero issues. Great power, idled smooth, ran smooth. Perfect.

When the symptoms started, it was still driveable, it just had the occasional miss. But, when I started it a few days later to wash it, it got progressively worse until it was sputtering with just a short idle and pulling it out of the garage. So, I reseated all of the plug wires and replaced the plugs. They all looked about the same as each other and looked fine. After I replaced the plugs, it started and seemed to idle fine for about 30 minutes and a little revving showed no issues. But, I didn't drive it. When I did drive it, it ran fine for about 10 minutes of idling and 2 minutes of driving. After 2 minutes of driving it started to miss and sputter a little, but after 5 minutes of total driving, it was barely staying running. I pulled over, checked the fuel pump, let it sit for about 5 minutes, and restarted it. With some gentle feathering of the throttle, I got it revved enough to limp it home.

I replaced the fuel pump and filter as preventative measures. When I went to start the car before the timing belt job, the fuel pump was frozen and took a tap to get it running again. So, I thought that might be a good thing to replace regardless. I've also pulled about a cup of fuel out of the tank from the same hose that feeds the pump and am letting it sit to see if there is water in the fuel. So far, it doesn't look like it. Replacing the fuel pump and filter had no effect.

What is the next logical step? Drain all the fuel and put fresh fuel in? Start checking connectors (which ones)? Is the computer suspect? Should I start with putting sensors on the plugs to make sure that they are firing? Will a standard noid light work on the 928 injectors? Or...what make more sense at this point?

Any guidance would be greatly appreciated. Thank you in advance.

No visible water separation after letting the gas sit for a couple of hours. There were a couple of flakes of black stuff in it though. Maybe 4 of them. I'm not sure if that's indicative of a source for this particular problem or not. But, I think that fuel is the most likely culprit at this point. My thought is to start by running a hose from the intake of the new fuel pump directly into a fresh container of gasoline. Just to rule out bad fuel. Assuming that doesn't alleviate the symptoms, I would like to confirm fuel pressure at the fuel rail. I do have a generic fuel pressure gauge, but I'm not sure if it will work on the 928. I can try it and find out, but if someone already know the answer, that would be helpful. If that shows a drop in pressure in sync with the stumbling/missing, then maybe I can assume a failed pressure regulator or clogged line. Otherwise, if the pressure looks steady even with the motor stumbling, then my assumption would be to start looking at the electrical side.

I don't know for sure when the fuel lines were last replaced. The motor was replaced by a shop in 2007 or 2008. I bought it in 2009. Hopefully the lines were replaced at the time as the motor, but I really don't know for sure. Maybe it would be a good idea to replace them now regardless. Is there a pre-built list of the parts appropriate for a fuel line refresh? Or, should I build it myself from the parts manual?

Start at the base for electrical issues: clean and tighten the battery connections. I am not sure if the main electrical feed runs through te starter motor connections. If so: clean and tighten also. Do you have a 14 pin connector on the right inner fender? Inspect for bad or corroded contacts.

I finally had a little time to mess with it today.

First, I ran a fuel hose from a fresh container of premium grade gasoline to the fuel pump intake. It did seem to make a little difference in how it was running. Altogether, it sucked about 3 gallons out of the container while testing it.

Next, I hooked up my fuel pressure gauge to the passenger side fuel rail. The port was capped with an NTP compression cap and a steel ball bearing in the cap. And it was tightened to a degree that seemed extreme to me. It took a lot of force to get it off. And the threads on the port are all buggered up from the cap that was on there. So, it looks like I'll need a new fuel rail. Photo of the cap and ball bearing:

https://cimg7.ibsrv.net/gimg/rennlis...76ce3010a0.jpg


Anyway, I managed to get the pressure gauge connected using some hose and clamps. The pressure was initially about 54psi, and then dropped to more like 47psi as it ran more. I haven't looked in the WSM, so I'm not sure if that's normal or not.

As it was running with the gauge connected, I noticed that the injectors were not firing like I would expect them to. Mine are pretty loud and you can always hear them clicking, but they weren't firing frequently. Which indicated to me that the computer was not telling them to fire. So, I disconnected the two computer connectors in the passenger foot well and inspected the connections. The computer side of the connections looked fine, but one of the plugs had some corroded connectors. I hit them with some 5% deoxit and reconnected it.

After that, the car ran much better. Not 100%, but much better. I pulled the connector and did a little more cleaning, but that yielded no additional improvement. At this point, the motor runs about 95%. Still a little hesitation and missing occasionally. But, it idles normally and responds to application of the throttle.

My thought is that the computer connection was part of the problem, but that bad fuel may be the other part of the problem.

Is it possible that the computer is sensing knocking and therefor keeping the injectors from firing? I may look through the WSM and see what is connected to the specific connectors that were corroded.

At this point, it seems like it may be ok to drive. I'll take it out when time permits and see how it does on the road. And top off the fuel tank as well.

Any thoughts are welcome.

Has an intake refresh been done?
Has the LH computer been rebuilt?
Has the MAF been refreshed?
Has new plug wires been installed?
Have the caps and rotors been replaced?
Did you pull the coil wires off to inspect then ends for corrosion?
make sure the coil wires dont touch any part of their run.
pull the vacuum line off the front damper smell for fuel.

Stan,

Thanks for the suggestions. This is a car that you worked on in Salt Lake City, which I bought in 2009. You probably know much more about the car than I do :) It's the one that had the entire interior replaced with a full leather interior from a wrecked car in Europe, and had the motor replaced shortly before I bought it.

No intake refresh since I bought it, and the LH and MAF haven't been done either.

The plug wires, rotors and caps are the same ones that were on it when I bought it, but they look to be in very good shape with not a lot of miles on them. I did put new plugs in and I put an inline tester in with the plugs and they all seem to be firing properly. That doesn't mean that there isn't a problem there, but everything looked ok when I had it apart for the timing belt.

I did inspect the coil wires and connections and they appear to be ok as well. That was the first thing that I checked. Also, disconnecting any one of the plugs made things much worse, which implied to me that the plugs are firing properly.

I'll check the vacuum line on the front damper. If I do smell fuel there, I assume that the dampener should be replaced, but would that cause the rough running?

Based on the symptoms, do you recommend a replace-parts-until-fixed approach? If so, are you recommending that they be done in the order that you listed? I've only put about 10k miles on the vehicle since I bought it almost ten years ago (with regular start/idle/warm-up periods), and things look to be in very good condition. I would hate to replace things that don't need it if I'm able to confirm that they aren't the problem. But, I have no problem replacing anything that makes sense.

Thanks again for your input. I'll take another look at the electrical end of things to see if I've missed anything there.

For the intake fresh, is there a reliable way to determine whether or not that is needed?

V/r,
Chris

Update: I started it this morning and it ran very poorly. Which means that the new fuel and cleaned up computer connection really had no effect. As it warmed up, it ran better, but not 100%

Does that sound more like a MAF issue? I've never done one on the 928 before, but I assume that I need to pull the air filter box out to get to it?

Would it make sense to also replace the LH computer, even if it's good now, since it will probably eventually fail anyway?

Is it worth buying the "Spanner" diagnostic tool (http://www.jdsporsche.com/jdsdiagtool.html), if it's still available?

Diagnostic tools are always a plus but they are difficult to justify for sole owner use- great as a local community common asset. At least sharktuner tells us that the knock sensors and the hall trigger are healthy and at the moment I would suggest those have to be your initial suspects as a fault detected in just one of those will knock 6 degrees of advance off your ignition timing across the board and that is significant

The real question was...does that particular diagnostic tool provide definitive results on whether or not the MAF and LH components are good or bad? If so, it's worth it to me. Are you saying that the sharktuner does everything that the "Spanner" does plus more? I didn't think that was the case.

The effects of retarded timing is much different than the symptoms I am currently experiencing. So, I think that it's probably not the hall or knock sensors. Unless I'm missing something here?

I think that Stan had the right direction with intake and spark components. The MAF seems pretty likely to me at this point. And I've found a lot of other posts where others had similar symptoms and it turned out to be the MAF. And the MAF supposedly does fail in a similar manner to mine where it gets progressively worse.

At least it gives me a starting point. I can pull the MAF and test it.

Does that sound more like a MAF issue? I've never done one on the 928 before, but I assume that I need to pull the air filter box out to get to it? Yes rebuild the MAF

Would it make sense to also replace the LH computer, even if it's good now, since it will probably eventually fail anyway? Yes rebuild the computer if it has not already been done.

For starters, I would drain the tank and get fresh fuel in there and see what happens - you may get lucky with doing just that.

You can easily drain the tank by removing the cap and steel ball from the passenger fuel rail, securely attach a long hose to a container, and jumper the fuel pump until the tank is empty.

Mine's an S3 ('85) so take this for what it's worth, but an intake refresh fixed everything you've mentioned for me. I did use MAF cleaner and I used Ken's Blink'r tool to adjust it according to his instructions (even made one and the special tool that goes with it). Mine didn't need a new MAF. I wish I was closer since I have a spare MAF could lend you now. John Speak has a business associate in Oregon I think that will rebuild yours without having to send it to England, I think you can find him on the web, I didn't have to go to that extreme, at least not yet. I also cleaned all my injectors and I found one beyond cleaning so I swapped it with a good one from my parts car.

It runs like new now. I couldn't say exactly which change fixed it. I replaced the idle control valve, cleaned the MAF, rebuilt (new seals, pintil caps, filters) the injectors, replaced one injector that wasn't firing, replaced all the vacuum lines and the vacuum distributor, new plug wires, new coils, new distributors and new plugs. When I was done it ran like a top. Didn't need to replace any of the computers. Most of that was done under Stan's expert direction.

PS: FWIW, pay special attention to the O ring that seals the MAF to the throttle body; mine slipped off and fell into the throat of the throttle while I was re-instaling the MAF. It caused all sorts of problems until I figured out what I'd done. Just a heads up?

Regards,

Thanks, guys. That helps a lot.

SwayBar, I ran it off of a fresh jug of gas yesterday with no change. So the easy route is out, unfortunately.

My plan is to remove the MAF and have it rebuilt. If that doesn't fix it, I'll re-evaluate.

If the MAF fixes it, I'll plan on replacing the LH, caps, rotors, wires, coils, and an intake refresh for 2019. I'm in the middle of a large project that I won't be done with for another couple of months, so I don't have a lot of time to work on the 928 right now.

Stan, my A/C is a bit weak on the 928. What are my options to get your assistance with it next year? Would it be necessary for me to bring it to you and leave it, or are you able to come my way to work on it? I have a lift, air, tools, etc, if that makes a difference.

ST2 does not have the diagnostic capability of the Porsche units but for those of us who use the thing it give us a lot of insights into what is happening within the engine and we can tell if MAF readings look suspiciously low, we can tell that the engine temperature sensors are working as well as the Hall and knock sensors and more,

The typical age related issue with the MAF is that it under reads the amount of air so performance suffers a bit but other issues can and do manifest themselves. If someone tells me that their S4 is underperforming a bit then I immediately suspect the sensors or the MAF. If the engine is misfiring or hesitating then any number of things can be going on. Stan has given you a number of suggestions to eliminate, best way to eliminate the MAF and computers is to fit yours into a known good example and see if the issues migrate with the changeover. Louie up in Oregon can test the MAF and the LH for you for a nominal fee and they can be rebuilt if knackered. The injectors are also quirky things and can suddenly go pear shaped even after cleaning and testing suggested they are fine.

I am very much against blindly splashing out on new or refurbed kit blindly but sometimes it is the only way if one wants to ensure reliability- that or do as I do and carry some key spares when I go on a longer journey [rare these days]. Other things to look out for are a damaged MAF connector under the boot, earth straps in the rear hatch and engine bay, wiring in the engine harness & temp 2 sensor.