Jim M.

I've got a problem that I can't seem to resolve and now I need the combined knowledge of Rennlist. HELP PLEASE!

The car: 93 GTS 5 speed, 112,xxx miles.
The car has an oil temperature and vacuum gauge as well as an ARM-1 air fuel ratio indicator. (see avatar) GT cams, 24 lb. injectors and new knock sensors less than 2 years ago. X-pipe and 2.5” mandrel bent exhaust system from Motorsportincslc. Shark tuned, using PEMS; dyno’d at 339 HP on a dynojet, approximately one year ago. Rebuilt MAF, new distributor caps and rotors, new coil within the last 60 days. New crank angle sensor and internal and external fuel pumps approx. one year ago. New O2 sensor less than a year ago.

The symptoms: low speed drivability has deteriorated all the time. When on the highway it will bog down and run rough. Often happens just as the flappy is due to open (coincidence???), other times it happens at a steady throttle setting. Takes almost full throttle to just maintain speed at times. Also very erratic, sometimes will do it every 15 seconds sometimes 30 minutes between problems. At times shutting it off then restarting either by the starter or popping the clutch will reset it and it will run fine for a period of time. Other times nothing seems to work. Hot or cold doesn’t appear to make a difference. Today on the first drive it wouldn’t idle at all, drove it for approximately 30 minutes and it idled at 300 RPM. When it is in the bog condition the ARM-1 will go completely dark or only one of the red (lean) LEDs will be on. Vacuum appears normal given throttle position when trying to maintain speed or stopped at a light. When it’s operating normally it’s as fast as a bullet and all vacuum, oil and water temps are normal. Voltage gauge also is normal, above the 12V line. The fuel pressure gauge on the RH fuel rail is also normal. Everything else on the car works as it’s supposed to.

Troubleshooting steps so far:
1. Swapped the PEMS for the stock chips – NO HELP
2. Swapped the LH with stock chips with a known good unit – NO HELP
3. Swapped the EZK with a known good unit and stock chips – NO HELP
4. Swapped the ignition module (front of radiator) with a known good one – NO HELP
5. Put the hammer on it and no faults found. All checks were good.
6. Reinstalled the PEMS in the original LH & EZK – NO HELP
7. Checked the 4 cylinder relay for a red or green LED’s, all off during the problem.
8. Replaced both coil leads with factory units. Checked the spark plug leads with the windex spray test in the dark. No fireflies or other indications.
9.Cleaned ground on the LH side of the block against the firewall, cleaned the chassis to engine ground, cleaned the ground at the ignition module (front of radiator). The ground above the CPU panel and the battery ground strap to chassis was cleaned about a year ago but I will clean them again. Both battery connections are clean and tight.
10. Verified the flappy is working, opening and closing a full 90 degrees.
11. Checked for frayed wires at the MAF connector.
12. Checked the MAF boot and all hoses and vacuum lines (Vacuum is normal on my gauge).
13. Car off, 12.58 VDC at both the battery posts and at the jump post. Car running 13.56 at the battery and 13.55 at the jump post. (It’s now idling at about 700 RPM more of the erratic nature.) The battery is 3 months old and changed due to age. There is no battery drain problem.
14. Checked the 14 pin connector all clean and shiny. Sprayed it with contact cleaner just to be sure.
15. Swapped the LH relay with the horn relay – NO HELP!

I'm open to all suggestions. It has to be electrical or the resets wouldn't work, but what could it be? If I was loosing ignition I think the 4 cylinder mode would kick in becasuse of the loss of heat.

ZEUS+

Possible MAF failure. Disconnect the MAF and see how it is.

Tom. M

check temp II?

White Lightnin'

One of the final stage modules?

Not very common, but happened to me. It would run rough periodically... each time getting longer, until the bad module finally gave out totally.

Do you smell a strong fuel smell when it starts to run rough? If so, you are probably only firing on half the cylinders and raw gasoline is being dumped into the exhaust. If you have cats -they can catch on fire & burn the car to the ground.

Rob Edwards

Agreed on bypassing the ICM relay, temporarily.

CE panel cleaned recently? (Ever?)

Relays V, X, XXV, and XXVI all known good? Buying seven new 53B relays from Roger is $35. Relay^3!

Longshots:

1. New CPS starting to fail? Dunno whether the Hammer has a fault code for CPS.


2. Did you R&R the fuel filter and blow out the connecting fuel hoses when R&Ring the fuel pumps? Maybe there's a chunk of old short hose intermittently blocking fuel delivery? You said fuel pressure is stable at the rail- that's while you're standing there watching it idle, but is it stable when the car's misbehaving?

928er

have you checked the in tank pump or the external pump? I know you preplaced them about a year ago but the in tank pumps are known to have a short life....

Jim M.

When we tried the spare LH it also had the 4 cyl. relay on it so that was also eliminated.

The problem existed before the new MAF. I doubt I would get a rebuilt with the same problem.

Temp II will effect starting but IIRC not my symptoms. But I'll check it.

We tried a complete known good final stage with no change (item # 4)

Lastly, I've never had any smell of fuel during or after the problem occured. Keep the ideas coming guys!

ROG100

On the list ---
Checking coil grounds for continuity
Checking grounds at rear of engine below MAF
Already has a rebuilt MAF
Swap manual gearbox for a decent auto - just kidding.
Swap out the relay on the brains - have a spare.

hans14914

Try jumpering the fuel pump (directly from the fuel pump to the battery). I havnet seen it on a 928 personally, but helped a local 944 guy with a similar problem. There was a fault somewhere in the fuelpump harness - never found the problem, we just opted to relay the pump at the pump.

If you do hotwire the pump, and you have a good DC ammeter, check and see how much juice your pump is pulling. This will tell you a lot about what is going on, and if the problem is either pump. If the intank pump has failed, it will put extra strain on the external, which you will see in the amp reading.

Bill Ball

Jim:

I don't know if the ARM-1 is giving you useful information, but it seems to be indicating you are going lean. A wideband that gives a direct AFR readout would answer if this is fuel related. Bogging due to going lean would be consistent with what you report. A bad throttle position sensor or its harness plug might do this. If you have or can borrow a Sharktuner, you should log one of these events. It would show if the throttle sensor is going to idle at the wrong time. Bad O2 sensor?

If I help solve this, then you should help me with my high speed vibration.

SeanR

On the TPS, new last year, harness good. Tested with the Hammer also and tests good several times for Idle/WOT contact.

This does act like an intermittent 4 cyl mode, such as the relay (tested with another, not jumpered), or a bad coil wire or coil wire connection. We have literally changed out dang near everything you can get to in a day with known good units from either my car, Rogers car or new in stock parts.

Last night Jim went though and double checked the grounds at the body and engine with no change. It's not an over fueling issue, it might be a lack of injector pulse but it's not happening when sitting idle so real hard to test.

Jim mentioned something yesterday as we were test driving, the car apparently has always had a slight stumble from take off. When it was doing it yesterday it literally felt like it was missing, coughed, and then caught and went vroom. I had the car in my possession for a few days and did not encounter anything like what we had going on today and I'm really thrown for a loop. I'm going to grab a few sets of coil wires and have those stuck in just in case it's an intermittent contact in a wire. The starts and idles fine so it's difficult to test w/o a test drive.

This seems to only happen when moving or under a load, which is making me take guesses. The harness might have a bad wire in it, hell, right now we are just grasping at straws.

Stromius

How about cap/rotors? New enough?
Temp II and MAF wiring/wiring boot ?

Landseer

Out there, but could it be possible for an injector to short internally? Or to have one bad one?

Even if new 2 years ago, reading between the lines, this still might be worth checking or swapping.
Not sure if data logging is possible. Or if only one bank could be affected by this on GTS.

Tom. M

Did you try moving/shaking the injector harness on both sides to see if it changes the idle characteristics?

Bill Ball

OK, so you've replaced everything. You still have no good information as to whether the events are fuel and/or ignition related. I would feed a wideband to a Sharktuner and see what it shows during one of these events. The wideband, even w/o Sharktuner, will show you if you are going lean. I know it's not exactly simple to log with Sharktuner if the bad events are very sporadic and you are driving around alone.

It could be injector-related, but know that a short anywhere in the harness takes out all the injectors as they share a common ground. However, one problem I've seen a lot with the factory injector harness plugs is loss of the wire retaining clips. They tend to fall off and the plugs are free to lose contact. One 87 I worked on had 5 missing clips.

I suppose you could have an intermittently bad coil and not have it picked up by the ignition monitoring system, but I think that would be odd.