Help Diagnosing strange hesistation/surging

triwave · 05:09 AM

Hi Guys,

Background: 1985 Carrera 3.2L mostly stock, few tweaks like a autothority chip and Magnecor wires. Car was smogged (with Cat) and passed with flying colors so I took the Cat off and replaced gaskets and O2 sensor. Was running great ... except...

Problem: Noticed under full or near full throttle acceleration in 1st gear that there is a strange surging. It's hard to describe but it's like a loss of power. No real funny sounds, just a surge in power. Backing off on throttle solves problem. It almost feels like traction control and kind of correlates to when the wheels are near to breaking traction. Wierd.

Additional Info: If I let the revs rise in 1st gear but without punching it the problem doesn't seem to happen. If I punch it in any other gear or lets the revs climb it doesn't seem to happen. Only 1st gear. Runs smooth and clean in general, nothing else problematic happening. Drives and shifts fine in general if you avoid very hard 1st gear acceleration.

I have mechanical aptitude in some areas, but drive line stuff is not one of them. My car has a LSD from the factory. The problem seems to correlate with torque. Could there be something going on in the drive train that might cause this? The LSD? Something clutch related? Transmission related? Can seem to draw any engine related conclusions as the problem isn't repeatible with certain throttle positions or RPMs...

Looking forward to thoughts and suggestions from the many experts on this forum

Thanks

Warren

Edit: One other comment. I have not driven the car much over winter and can't say that the surging I am feeling has anything to do with the swap of Cat converter with test pipe and a new O2 sensor. Probably just coincidence that I started driving it more. I can say however that I have a common problem with my idle hunting a bit on cold start-up so I though I would change the O2 sensor at this time to see if that would help any. (it didn't)

stlrj · 03-20-2005, 06:17 AM

It sounds like an ignition related problem. I've had problems with Magnecores in the past that were brand new and yet resulted in a miss and studder.

-Joe

triwave · 03-20-2005, 08:53 AM

I tried to rule out ignition problems thinking it should be repeatable at similar RPM and throttle positions. I only see this problem in first gear. Is there any kind of gear sensor on the transmission that would be smart enough to change fuel maps or something depending on gear? I have not hear of that, I only hear people talk of the ignition/fuel map...

cyprusx · 03-20-2005, 09:10 AM

Clutch?

triwave · 03-22-2005, 06:04 PM

I've driven the car a bit more ... problem persists, but I'm feeling more and more like it's not mechanical drive train in nature. There are no loud noises or scraping or binding or any feelings like that. The tach bounces up and down a bit as you hit this "surge". Can't imagine clutch slipping and grabbing so rapidly. I could see slipping, or chatter, but this is after clutch plates are fully engaged.

Joe (stlrj) mentions the Magnecors which is intriguing but I'm still trying to connect in my mind how this could only be a first gear problem. Any theories? I honesty cannot get similar effects in 2nd or 3rd and I'm risking tickets driving around town with full on throttle all the time ... though it's kind of fun

I started thinking about fuel supply as it has that loss of power sort of feeling. Anything possibly related to fuel pump, senders, injectors that could have correlation to this sort of problem?

I'm not too sure where to begin so I'm still interested in any and all opinions !

Thanks,

Warren

stlrj · 03-22-2005, 06:14 PM

Warren,

What was done prior to this surge? I assume that at some point there were no problems and then something was done and then the problem.

Sometime a little detective work can save you a lot of time and frustration.

Cheers,

Joe

BURN-BROS · 03-22-2005, 06:21 PM

Check for any vacuum leaks after the air flow meter. A hunting idle can be the result of that, possibly the hesitation your experiencing as well.

triwave · 03-23-2005, 04:07 AM

Joe,

Car has had various stages of work done. Last fall I had the car tuned up to pass NC inspection. I got a valve job, plugs, mixture adjust, idle set and had the cat converter put back on (using same O2 sensor).

Didn't drive the car much over winter and in general drove it easy so if the problem existed I may not have observed it - you really have to punch it in 1st to feel the problem.

More recently I took out the cat, put back in a bypass pipe, and a brand new Bosch O2 sensor and gaskets. The car was running so nice after this I started driving it a bit harder and then observed the problems mentioned.

I'm think if swapping back to my old O2 sensor ... kind of a long short but it's so simple to do it would take out one variable.

Vacuum leaks are a good idea. How does one really go about looking for them?

mede8er · mede8er's Garage

Sounds like a fuel prob....have you checked your fuel pressure.....

BURN-BROS · 03-23-2005, 04:58 AM

Just check vac hoses for cracks or other damage. Check the rubber boots between the air meter and the intake manifold. Usually a hunting idle is the computer compensating for a lean condition. It then becomes too rich and corrects. This cycle gives you the pulsing of the idle.

Check for a binding air meter. Manually push the door and note any binding.

triwave · 04-16-2005, 08:34 AM

I'm going to look at the mixture again. It smells a bit rich, especially when cold and idling. Applying gas off idle it's possible to get a real hesitation and almost stalling. Once the oil is fully up to temp the problem becomes hard to replicate, but at WOT when I lift off the gas to change gears (at redline) I occasionally get a small backfiring. This makes me think I'm running rich overall, and it's just less prevalent when engine is hot.

I've read about the mixture adjust screw, do this vary the mixture across all temps and RPMs or is it more just for idle? i.e. trying to figure out what has most effect on mixture when engine isn't up to full operating temp ... and what adjust mixture at idle vs. higher RPM

Thanks

stlrj · 04-17-2005, 06:46 AM

The mixture screw has ifs greatest effect on idle, very little higher up.

But before going to the trouble of removing your air flow meter to drill out the anti tamper plug, try driving with your oil cap remonved and see if that improves anything.

Cheers,

Joe

triwave · 04-18-2005, 03:42 AM

OK, I can do your suggestion easy enough ... but please explain, what is the link to oil tank cap and mixture? Thanks!

stlrj · 04-18-2005, 09:31 AM

When you remove the oil cap you are leaning the mix the same way the CO adjust screw on the air flow meter allows air to bypass the barn door.

triwave · 04-18-2005, 10:13 AM

That's why I love this board, you learn a new thing every day! I can certainly try this while idling in the parking lot to see if that improves the rich smelling exhaust. If I actually drove the car that way would oil slosh out the top or is there some sort of baffle or something that would prevent that?

stlrj · 03:15 PM

You could always install a K&N air filter in place of the oil cap which, in you case, could result in a performance increase if your engine was too rich.

Or...you could place an old sock over the filler neck.

Sorry, I've been watching too many MacGyver reruns.

flynnstudio · flynnstudio's Garage

I get it on a gas anylser to check that mixture and check your DME switch - usually find some-previous-anonymous has gone and advanced the timing or enriched the mixture 'the quick and cheap way' instead of fixing 'tired' ancilliaries...

jester911 · jester911's Garage

Triwave,
Have you solved this problem yet?
I am having basically the same thing after swapping to a euro
premuffler and sport exhaust.

After doing some extensive searching on this board I think I have some insight, but I am going to try and get to the bottom of it today.

ruf-porsche · ruf-porsche's Garage

Check my response in this thread

Stumbling at low rpms-'88 3.2 Carrera