GregBBRD

Keep in mind that not running the O2 sensor allows the engine to run richer than with it hooked up...and you say this helped. However, reading back, increasing the amount of fuel injected by changing the spring on the door did not help (actually made it worse, as I recall).....note this also richens up the mixture.

Tough, for me, to make these two facts equate.

At any rate, from your last post about the intake leak, you have an obvious cam carrier to cylinder head gasket failure. I thought you were going to remove the intake, cam carriers, and heads once the show season was over?

I'd certainly suggest starting with fixing the obvious and hope that you find the running problem when you do this other work.

granprixweiss928

Greg

thanks for your response. Yes I agree its been confusing.

with the 02 unplugged and the adjusted spring on a rebuild AFM, I am getting a nice consistent 14.6-14.7 AFR when driving/cruising.

Yes the plan is to pull the engine and do the head gaskets/cam covers- this spring. My fall/winter work schedule does not allow for car work (and Ive been playing with my new-to-me 997).

granprixweiss928

Well I think I have this solved.

I took a chance and picked up used ECU for a good price off ebay.

I figured if, if the new ecu nets the same results then I know for sure its not the ecu in the car, and then Id have a spare on the shelf. I had tried another ecu before, and it didnt solve it. I wanted to try another.

The new Ecu is the later version, 04 from and 84, while the original one in the car is an 02.

I put in the new ecu, reconnected the O2 and took it for a nice spirited drive. It felt way better! Acceleration was smoother and it seemed happier at idle as well. exhaust smelled less rich during warm up.

I then put the original Ecu back in and felt the same issues as before.
switched back to the new ecu tonight and drove it again and its way better!

The Deputy

Good news...I hope the fix sticks.

Brian.

Brian'sIris

This guy is a Spammer!!

On topic: Glad to hear your problem is fixed! I have a similar problem with my '85 5-speed. Bucking in cold weather and when engine is cold, goes away when accel. pedal is floored. Bucks violently and rapidly at times...must be hard on the drivetrain! Other times just a gentle bucking while cruizing.

Hoping my (ongoing) intake refresh, etc. takes care of it, but will look here for clues if not.

Cheers,
Brian

Hercdriver

I am trying to solve the same problem on a 86.5. The bucking only happens when the car comes to a stop. The car runs smooth otherwise and no bucking while on park or neutral. The bucking is more prominent when the engine is cold.

Thanks in advance

Speedtoys

What maintenance has been done on it that you know of?

granprixweiss928

Let me follow up here with my issue.

I just finished a complete engine out drivetrain overhaul this summer....

https://rennlist.com/forums/928-foru...uild-pics.html

and while the rebuild found issues- like heads with excessive worn guides dumping in lots of oil, and creating lots of carbon build up-

Once all back together again the issue is still there. Its been determined that its the way it is- the fuel management system is an early design and analog(?) and 30+ years old. I have settled that its just the way it is.

and- with the O2 unplugged, its still there, but way less so. but then it stinks when its cold and warming up.

Hercdriver

Hello speedtoys,
Maintenance performed:
Sparks plugs/wires
Both coils.
New MAF(re-built)
I just moved from DC and on cold mornings it was more noticeable, now in Florida is not as severe as in DC. I need to change all vaccum lines and the THrottle position switch. I dont know if I have an iddler valve and location, need to do some more troubleshooting, still learning this engine.

bureau13

I know the Temp II sensor tells the computers what temperature range its in, so that could be related here. I just ordered one because I have some occasional cold running issues, and because it's the original one (I suspect) and could use with a change. There's a test you can run with an ohmeter but in my case, it's intermittent.

For what it's worth, my original "engine bucking under light throttle" problem was injector related. I'm not exactly sure about what you're seeing though, you mentioned it happens as you come to a stop, which is different.

Mrmerlin

I would suggest to install a new green wire this was discussed on page two,
but I never saw it noted that a new wire was installed. deox100 all connections.

Remove the connector for the computer in the engine bay and put deoxit 100 on the connector,
then plug it together.

Remove both of the ground wires for the injector system from the right cam tower,
and then remove any brackets and clean them all,
then add deoxit 100 to the connections .

Replace the temp 2 sensor connector and put 100 on the temp 2 sealing ring then tighten it up.

Remove the distributor and on the bench verify that the vacuum advance unit works smoothly.

Use a feeler gauge to verify that the fingers have clearance both while advanced and not advanced.
make sure none of the fingers are bent.

Verify that the roll pin that locks the finger plate to the shaft is still in the groove.

Remove the computer connectors inside the car at the pass footwell and put deoxit 100 on them.

Go to the hatch and find the ground bundle in the RR area of the hatch floor or behind the RR quarter liner loosen the bolt then put deox100 on it, tighten bolt.

Remove both battery terminals and 100 them. same for the ground point connection.

Replace the engine ground strap to chassis. (this step might have been done.)

Replace the O2 sensor with a new factory part.

Put 100 on all of the spark plug wire connections and the coil connections.

Test the coil to see if its performing as it should see WSM.

Please answer every step as I have suggested,
NOTE even if you did it last year follow these suggestions,
please report your findings

mkriete

Thanks for the update. I've had a bucking issue at high RPM under load and have been watching this thread closely. Sorry you have not completely solved your issue.