308 QV intermittent mis-fire

I hit a bump last weekend, car immediately started to misfire/stumble. A minute later it kicked back in and ran fine. It's done it again a few times since then, usually from a stop or from idle, but then something connects when the rpm's are up and again it's fine. Doesn't happen every time I drive it. Filters are all new, new plug extenders yesterday and though I had it - until today. Nice when a V8 Vantage pulls up next to you and you can barely pull away.....

 

PS, any ideas? Replace the rear valve cover gasket, but that was ~500 miles ago. Injector? Emissions - hose loose?

 

What year/version 308QV?

 

I made a post a while ago about someone in a 328 with similar problems. Here's what worked in my '84 QV:
-possible loose wire under distributor, try pushing up on the wires. when the car gets older, even new wires can slip a little off of the distributor (do your RPMs drop to about 500 for a second or two?)
-there is also a rubber-booted clip going to the fuel distributor (coming from the rear of car) that can become corroded or loose, and thereby shorts-out the air/temperature adjustments (does your car idle near normal RPM but become very rough/jerky after the bump?)

just my best guess.....

 

Don't forget the flywheel sensors. You may have one beginning to crap out on you. I just went thru this and it started out exactly like yours. A little bit at a time. Watch your tach. If it dies while it is acting up it let us know.

 

Also the fuel pump relay/connection/whatever it is by the fuses. when it's acting up again first pay attention to the tach and see if it dies. Next pull the fuse panel off and see if the fuel pump hook ups are as hot as a firecracker. If it is, it's that.

I had this happen about 8 years ago

 

My situation was very close to what yours is. I was playing with flywheel sensors, making sure all of the wires at the coils and the two large interconnect plugs were good, etc. On a test drive one day after doing some electrical repairs / troubleshooting, I accidently ran over a rough patch of road and the car began to run rough and miss. Must be a loose connection someplace.

I rechecked everything and just for grins, I checked the battery connections. Well, I found that with my Optima battery, the battery clamps were as tight as they were going to be but still loose on the battery posts. I found some battery post shims (looks like a large lead thimble), installed those and then reattached the battery cables. No problems since.

 

Great tip. I have an Optima too. I am checking mine tonight.

 

My car left me stranded yesterday and i had it towed to Omega motorsports. Guy called me today and said it's gonna be $150 for the three turns on the wrench i had to make to your battery ground.

Feel like an idiot..at least not a broke one though.

 

I hope you were stranded while it was parked; if it was running you likely fried the VR. If the ignition light starts lighting up, that's the culprit.

Ken

 

It was parked..all is well. I was at Mcdonalds getting health food.

 

Who sells spacers for the Optma posts?

 

You guys are great - flywheel sensor is not what I would have duh, er, thunk of. You're right, when it mis-fires, the rpm's at "idle" drop to zip, I have to keep the acclerator pedal down to keep it running - revving keeps it alive and then it kicks back in and all is well. Until the next stoplight. If I keep blipping at the light like I'm driving a WRX then it usually stays ok.

 

P.S. Steve - it's an '85 GTS, 38k on the odo.

 

It is your flywheel sensor. Mine did the EXACT same thing. I was actually able to drive 200 miles like that from Atlanta to Birmingham by simply keeping the revs up above 3000 or so.

What will eventually happen is it will start cold just fine and drive away but within 1/2 mile from home it will die and no amount of throttle will get it back. Guess how I can be so sure...

You need to call T. Rutlands and buy 3 sensors. The one at the 11:00 position is the one you are having a problem with but at about $100+/- each you don't want to just replace one while you are in there. Then you call a tow truck and have them take your car to the shop. When the shop is done fixing it either you or the shop should change the oil.

Just don't drive it anymore, You are dumping raw fuel all through that front bank without burning it off and throwing it in the cat. That is asking for trouble.

 

If it's a US version (K-Jet with Lambda), you should also confirm/deny if the frequency valve is "buzzing" or not when you have the problem. If it isn't, you've lost the +12V power to run the injection system so it will run like a lean-adjusted K-Jet without Lambda -- and "blipping the throttle to keep running" is a known symptom

 

Don't forget he loses his tach too. It's that one thing that leads me to think it is the flywheel sensor. When I bought mine, they told me it was the 70-something one the sold just this year and that was in Apr.

Like a speedo sensor or a water pump, I think these are "whens", not "ifs", in the failure department on a 3X8/Mondial.

 

Yes, wouldn't disagree that "no tach at all" would be a different kettle of fish -- is he reporting that?

 

He mentioned it here.

 

I take that to mean that as it's stumbling at idle, the tach needle drops down (which is what is really happening -- isn't ~500 RPM where the needle rest is?). The better question is "what is the tach doing when he's blipping the throttle and the engine is continuing to run". If the tach is reading and matching the real RPM, I would not suspect the flywheel RPM sensor; if the tach needle stays "dead" down at the minimum value, I would suspect the Flywheel RPM Sensor (but the engine wouldn't run if that was the case) -- smss26 which situation do you have?

 

At idle mine would die. Just fall flat and not move unless the engine was throttled over 3000-3500 or so. Basically you would drive fine up to a red light and when you stepped on the clutch the tach would fall and not move again even though the engine was still running - on the front bank only. I would step on the gas again and after a few pops and backfires it would wake up again, until I tried to let it idle again. Thank God it got me back home like this.

Funny thing too; the engine would sound fine at idle while the hood was up and you were standing next to it. It sounded so normal that I had to pull a plug boot off to prove to someone that the entire rear bank was sitting there dead as a door nail. A 308 is a great 4 cyl at idle. Smooth as silk. It's really weird.

Anyway, I found the 11:00 sensor was shot. Since it has been replaced, I have put about 400 miles on it including a full day at the track. I burned a whole tankfull of gas at Talledega last weekend . All that and not another misfire. Not once. I am back where I trust my car again.

 

I went to the first auto parts store down the street and asked for "Battery Post Shims". They come in a 2-pack and look like a large lead thimble with a split down the sides so they can expand. It's a universal shim. I actually had to open my battery connectors to their most open state and using a rubber mallet, tap on the connector to encourage it over the shim. Worked perfectly.

 

Getting back to the original problem, you might also want to check the earth/ground connections for the ignition modules in the trunk. The plate they sit on is only earthed if all the nuts are in place holding it down. If they are loose or really rusty/dirty, then you might have a problem there.

Been there, done that.

 

Install the 10-guage ground discussed at length herein from the ECUs to where the coils ground.

 

the tach continued to match revs - stumble. Could it really have been as simple as the ground? Haven't had a fire since my '64 Vette 20 years ago.