dfinnegan

keywords: bucking, cut-out, missing, hisitation, poor idle

Hello old friends! I used to be a regular here, but a new job, and a growing family have consumed my time. I now visit occassionaly, but rarely post anymore. I miss this forum. This has always been a great crew, and a great source knowledge, comradery, and casual entertainment. I hope you are all well and happy.

Today I'm back looking for help, as I'm having difficult diagnosing an issue with my 1991 C4.

Over the past couple of months the engine has increasingly been cutting out under load, and more recently is failing to hold an idle and is stalling.

Symptoms:

- sometimes the engine will buck, cut-out, miss on heavy acceleration
- sometimes will not idle

Actions taken:

- new fuel pump
- new coils
- O2 sensor on order

Discussion:

The cut-outs started 2-3 months back. Very occassionally I'd experience an entire cut-out of power for less than a second on heavy acceleration. Generally in 2nd or 3rd gear. I'd instinctively pull off the gas peddal, the engine would catch and I'd accelerate off more gently with no issue. As the cut-outs became more frequent I started pushing through them trying to determine if holding the hard acceleration would change things. In these cases the car would buck several times in a second, or so, and, again, I'd back off. Under light load this does not happen.

At other times I can thrash the heck out of the old girl and come out of the hills and twisties with a huge smile on my face and be reminded of how much I love this car! Saddly, this is growing more rare. And the cut-outs are now occuring in every gear, though, still, mostly in the mid gears.

About a month ago the problem progressed to the point where it occurs on almost every drive, and is particullary bad after a long drive; 1 hour, or more. I am also having problems holding an idle. Sometimes on start up, but more often after a long drive. I have to heal-n-toe every stop sign and traffic light, and stop-n-go traffic is a real pain in the ****!

In the beginning I would sometimes see a check engine light when this occurred, but it would clear itself rather quickly. Recently I see the check engine light often when this is happening, and it sticks around for minutes afterward, but, again, no codes.

In fact, I have experienced a "no data connection" (or similar) fault when connecting the Hammer to try and diagnose this issue myself. (!) This feels significant to me, but reseating the connector has not changed the cut-out problem.

I've been to a shop and their advanced diagnostices computer based system could find no trace of the issue either. They gave me a strong recommendation to replace the fuel pump; which I did. No joy. I now have a spare - anyone need a replacement fuel pump?

I've had problems with coils in the past, and know the issues that have been discussed here regarding "Brazillian coils", so I went ahead and replaced the coils. Again, no joy. More spares - anyone?

I've also had O2 problems in the past and after 7, or so, years figured that might be worth trying, so I've ordered a replacement.

I know that I'm breaking all the rules on diagnosing problems - don't replace parts hoping to fall upon a solution!! However, after investing more time than I have freely available I'm getting desperate. I've read a lot of posts. I've double checked the AFM connection tech bulletin fix and added zip ties to all wires and hoses. I've checked connections for connectors on the O2 sensor, and a few other sensors. I've poured over the Hammer capabilities thoroughly.

I hate the thought of replacing all of my sensors (O2, CHT, Knock Sensor, etc), but am starting down that path in desperation.

So, if anyone has any input, advice, or suggestions, I'm all ears.

Thanks,
Dave

darth

How old are the spark plug wires in miles and/or years?

Vandit

Timely thread. I have been experiencing the exact same thing and it also got progressively worse. Mine however, didn't act up when driving hard, as i've done a handful of autocross events and it has always driven wonderfully. The problem would show up driving to/from the events or whenever i'd drive the car to work.

You can read more about it where I started participating in this thread.
https://rennlist.com/forums/964-foru...problem-3.html

So far i've replaced coils, caps, rotors, cylinder head temp sensor, fuel filter, dme relay, ran Seafoam in the gas, made sure the TSB on the AFM connector was performed, and the intermittent cutoff just kept on getting worse to the point the car wasn't driveable and even stall on me in traffic once.

Frustrated and no longer comfortable w/ driving the car, I pulled the car off the road a few of months ago and tore into the whole intake manifold assembly. I found cracked plastic intake manifolds and seals deteriorated so bad that they were falling into the head and showed signs of vacuum leaks where they meet the head (*more here*).

While I have the car is off the road, I am going to replace the ignition wires, spark plugs, O2 sensor, have the injectors serviced, and replace all the rubber seals and couplings. Fingers crossed that it makes the car run right again.

boxsey911

Dave, I have similar symptoms but not as severe. In my case it's the stalling issue when pulling up at junctions, especially when I depress the clutch pedal. I can fire it back up by letting off the clutch but then usually get a wildly fluctuating idle. I used to get the odd stall from cold because of the lightweight flywheel but it's now happening much more often.

I too think it's O2 sensor related so will be interested to hear what you find when you change yours. I've found that moving the O2 sensor wire inside the engine bay away from any plug wires has improved the problem but I don't have a reason why that could have an effect.

lushlife

Had similar issues- fuel filter was the cause. Jeff

JasonAndreas

Do you know anybody with a spare ECU? The only reason you should see the "no data connection" is when the RPM is above 2k (and that can be changed) Do you still come up this way? I don't have a spare but...

A faulty ISV will also cause your problems but that would almost always store a code.

Current Induction... That's why there are plastic spacers and brackets and clips all over the engine to try and keep the ignition wires away from the engine wiring harness. Porsche even had a TSB in the mid-eighties for it!

jimq

You dont mention changing DME relay. Could it maybe be a heat related intermittent problem with it?

ThomasC2

I've had problems with a "glitch"/misfire under full power passing 5000 revs. The problem was the generator going bad. Only charging 13,0-1. Doesn't explain your idle problems but have you checked that the generator gives 13,8-14,4 V? Otherwise the coils doesn't get enough power under maxium load.

Thomas

Simc964

I have exactly the same problem and it's driving me mad. Comes and goes. New fuel pump, switched the ecu and dme. Today cleaned up battery terminals. Each fix has improved for an hour and then it comes back. Hope one of us can find the fault so we can enjoy these great cars again. Will try the fuel filter as mentioned above next. I am rubbish with a spanner so each then have to take it in to the dealer, luckily it's still in its purchase warranty so seem to be back every few days!!!

dfinnegan

Thanks for all of the replies. I'll have to spend some time digesting, but for the quick answers:

yy.mm.dd mileage (current mileage 113k)
------------------------------------------------------------
13.08.21 110,100 replaced fuel pump
13.08.21 110,100 replaced DME relay
13.06.20 109,500 replaced a/c condensor
13.08.16 109,950 replaced rotos, caps, belts
11.03.13 88,510 replace spark plug wires
07.07.13 59,985 R&R injectors - WitchHunter Performance
07.07.13 59,985 clean intake, replace short stack seals
05.11.19 41,150 replaced distributor belt (still intact)
05.10.30 40,500 replaced O2 sensor
??.??.?? ? replaced generator (couple of years ago; someone here got the spare which we installed on the side of rte 84 near my house! though I failed to note my replacement)

I don't have a spare ECU; though I've been tempted in the past and wish I had "invested" in that some years back.

I did not know that the "no data connection" would show above 2k RPM. That would explain what I was seeing. This was while driving. I've run knock counts at speed and that worked. I figured the Hammer just worked at speed. Learned something new today. Thanks. So, how do you change that? Programming? I would love to have the Hammer work while driving and pull in the sensor data. I do still get up your way occassionally. Are you still in RI? Though, I'm hoping I can fix this issue soon and I'm not sure when I'll next be up your way. I was just there this week! Damn.

I will look into the current induction possibility on the left side of the engine bay.

I'll double check the generator, but I have looked at it recently.

Thanks everyone!

dfinnegan

Question: Can I run with the O2 sensor disconnected?
Any risk in doing so?

I figure it may run rough, but if it does not cut-out that would be a good indicator. (just getting impatient as the new O2 sensor is on order from Sunset and won't be here for another 1-2 weeks.

I love my daily ride, and really hate it when it's acting up. And, boy, is it acting up.

Cheers

jimq

will run rich. Not real good for the catalytic converter if you have one

ras62

What is the motor off battery voltage? Then voltage at idle?
You seem to have covered most points except maybe the battery earth wire replacement. Also worth checking the underside to make sure there are no problems with starter motor connections or short issues on the main 12v feeds between battery and alternator.
First check the AFM. I also thought my AFM TSB was sorted...it wasnt and the pipes had moved...maybe worth checking they are far enough away from the AFM wiring.

Frank 993 C4S

Crank Position Sensor

Goughary

I am with the crank position sensor camp...but as an aside, when I dropped the engine to do the top end a couple years ago, i found my engine wiring harness was a mess. Old frayed cracked and had all kinds of burned stuff going on. Casing missing etc. Not to mention that someone had done some shoddy repairs in the past. I fixed a ton of stuff when the motor was out...but I'm sure that quite a few of the disappeared gremlins were do to the renewed wiring harness that went back in...so as long as you are tearing things apart, pay attention to cranked housing on the wires and potential shorts and grounding issues