88 Coupe backfire troubleshooting (long)

[Stuttgart951]

Regarding my potential purchase of the car mentioned in this thread:

Going to look at a car this weekend

Id like some opinions on what may be causing the backfire mentioned. At this point, Im trying to decide whether or not a PPI is worth it to determine whether or not the problem is major or minor - or if I should walk.

Here are the details:

88 Coupe - Fired up just fine, high idle for 20 seconds, and then down to normal - as per the norm.

Car drove fine, with no noticable backfires when driving (underload, engine deceleration, etc - no problems that I heard)

Upon arriving back at the sellers home, the individual that went with me reported hearing several backfires (pops) when I came up the hill in 1st gear at about 3000 RPM. I did not hear these backfires while in the car - but apparently they were audible outside.

Ive done a search and am in the process of reading all the information that came up.

What Im looking for are opinions on what could be wrong with the car - what would cause the backfiring?

Before the car was purchased by the current owner, he reported a stumbling problem and had the chip replaced - this apprently solved the problem. He mentioned nothing of a backfire.

Im aware that a mechanic could probably sort this out - but what are the best and worse case scenarios here? Id dont want a car that will require $10K in work immediately following purchase.

Could it be a DME problem? Fuel delivery? The need for a valve adjustment? Car is totally stock (replacement chip is factory).

::fingers crossed::

Thanks.

Mike

[Rot 911]

Did the backfire occur while you were still accelerating or when you let off the throttle? My guess if it was when you let off the throttle is that the after market chip has your car running a bit rich and the excess gas fired off in the exhaust/muffler. If that is the case not much work or $ to fix.

[RickM]

I think Kurt's suggestion makes sense. This happened to me after installing an Autothority chip on my '85 Carrera. But I also had the Throttle body enlarged by CMW at the same time. SO......I can't tell you that it's one or the other or both. I can tell you that it seems to backfire very rarely, but that may be because, as you say, it can't be heard from inside the cabin.

Rick

[Stuttgart951]

I was not accelerating when the backfires were heard outside. I was progressing up a rather steep hill in first gear at roughly 3000RPM. No accleration, no decceleration - a steady speed.

Im under the impression that the chip is not aftermarket - I am relatively sure the chip is simply a factory replacement.

Everything Ive been reading for the past 2 hours gives two viewpoints.

A.) This should NOT be happening on DME controlled cars

and

B.) This happens - dont worry about it.



I dont mind a little bit of work - but Im in no hurry to rebuild an engine - and of all the 2.7s, 3.0s, and 3.2s Ive driven, Ive never experienced this problem and its making me nuts... the car was perfect in every other way... I probably would have jumped on it and sent it to a mechanic immediately if not for the backfire...

Keep those opinions coming.

[RickM]

Mike,
Is the car a daily driver or does it sit in the owners garage for extended periods of time?
Rick

[Stuttgart951]

Rick - its a second car.

PO put 10K miles on it in 3 years of ownership.

[nostatic]

it's times like these that I'm glad I own an SC.

Well, actually all the time I'm glad I own an SC

If the car drove better than others you've checked out and it seems clean, I think it is worth having the PPI done. This backfire doesn't seem like a big deal and if it is the PPI should catch it...throw some Techron in and drive it every day.

[RickM]

Beacause of too many toys I put alot less than that on my car (looking to change that soon). I'm tempted to say that may have some bearing on the situation. Wonder what John Walker or other more knowledgable members have to say.....

[Stuttgart951]

Aoy... first of all, I really appreciate you guys helping me out... if the car was total junk I would just write it off - but other than the backfire issue, its damn near perfect.

Secondly - Ive "discovered" something new that may change the situation...

Apparently (as you recall, I couldnt hear it in the cabin) I was wrong about WHEN the car was backfiring.

I understood my buddy to say that it was poping while I was driving in 1st at 3K RPM - but apprently it backfired when I let off the gas in first to park and then continued to backfire a few more times when I was still in the car (doors closed) during idle. Im sure this alters what the problem may be - but Im still a bit confused as to what the possible problems are.

Part of me is saying exactly what some of you are - put some fuel cleaner in and get the cobwebs out of the car (I plan to drive it about 10K a year - basically whever the weather is warm - so Ill be driving it much more often than he does) but the flip side is that I want a dream car - not a nightmare... and I tend to be pretty anal about cars - as Im sure all of you are.

I figure Ill sleep on it and decide whether or not Ill pursue a PPI in the morning. If I go through with it and it turns out to be something simple like a relay or switch, Im game... but I sure as hell dont want to go through the endless process of guesswork diagnosis (ask me why).


Keep those opinions coming - Im sure Ill be hitting the refresh button more than working tomorrow at the office.

[Rot 911]

Originally posted by Stuttgart951

I understood my buddy to say that it was poping while I was driving in 1st at 3K RPM - but apprently it backfired when I let off the gas in first to park and then continued to backfire a few more times when I was still in the car (doors closed) during idle. Im sure this alters what the problem may be - but Im still a bit confused as to what the possible problems are.

With this info I will stick with my opinion that the aftermarket chip has the fuel mixture running rich.

[tshih]

Take it to a known good Porsche mechanic for a PPI and have them check the cylinder head temp sensor, compression, leak down test, exhaust gas analyzed for richness and CO (emissions' testing anyway!). You could always not buy the car if major problems discovered or get further price breaks from seller. Probably idle stabilizer and source of air leak in the intake system should also be checked because backfiring is possibly caused by leaness (air leak after AFS in induction system), certainly bad fuel mixture, or poor fuel run-on due to carbon deposits.

[Stuttgart951]

Hmm... this would also explain why the car felt slow to me - maybe it wasnt just my imagination.

If we go with the assumption that the chip is aftermarket - and running rich to cure the stumbling he said he had prior to his purchase - I can remove the chip and replace with another - but if I go back to factory Im back with the stumbling problem... best to take it to a good shop I think to sort it out.

Im pretty firm on where I would like to take it for a PPI - I really trust my mechanic - hopefully he will be ok with that - if not - I dont think I would be confortable with the car.

It would be great if I could just have 3-4 hours with the car to myself so I can look at EVERYTHING... check for intake leaks, exhaust leaks, etc. There are so many things that I woke up this morning kicking myself for not doing yesterday.

[masraum]

if it is due to a rich running problem it could be a leaky injector too. And then as others have stated there's always the possibility of a vacuum leak.

It shouldn't do that, but it's probably not a big deal if it ran well. BTW, it's been my experience that a 3.2L car should feel faster than an SC. I drove both when I was looking, and the 3.2's felt much faster. I suppose the 3.0 could all have been in need of some TLC which is part of the reason I didn't buy them.

[Stuttgart951]

Slightly OT - the car was serviced at "Edison Motorworks" in NJ (not exactly sure where - assume Mercer county). I cant find a webpage after searching - or a phone number. Anyone familiar with this shop?

Id like to talk with the mechanic that worked on the car before I take it to my guy.