Engine miss after new exhaust installation
 

I've been using a europremuffler for some time with no problem and recently installed a Fabspeed 1 in 2 out muffler. It is a very non-restrictive muffler. Early on I noticed a barely perceptible miss when starting off in 1st gear. The miss seems to be progressing and today it missed at about 6k rpms (as well as stumbling when starting off at low rpms). Although the problem is intermittent, it is becoming more regular. Also seems worse when engine is warmed up. There is also a burbling/backfire sound when I lift off the gas...(not a bad thing in my opinion, but might be related).

Could it be related to my chip? I've ordered a custom chip from Steve Wong which is in transit, but was wondering if this could be the problem. My current chip is probably not mapped for a free-flow exhaust. I'm wondering if the car is running lean (definately not running rich as there is no smoke what-so-ever).

When it cools off, I'll check to make sure all plug wires are tight.

Seems awfully suspicsious that it has only started doing this since the exhaust was changed.

Thanks for your help.

My car did this for a while too after I fitted a B&B exhaust. I think the chip remapped itself after a few days of spirited driving though, since it doesn't seem to miss at all now.

Hmmm. If that's the case...would it help to disconnect the battery for 30 minutes to "reset" things?

Mike, maybe you should call the very helpful customer service professionals at Fabspeed. I'm sure they'll be more than happy to help you out! javascript:smilie(':rolleyes:')

Do I detect a note of sarcasm in your voice? Naaaaaaah.

Checked all my ignition wires and everything seemed tight. Gave em' all a good wiggle anyway...drove home and it ran perfect! Just for grins, I disconnected my battery and will hook it back up and go for a spin in about an hour.

The '84-89 DME are not adaptive. Disconnecting the DME will do nothing. Let us know what it turns out to be.
-Chris

Thanks Chris. Went for a spin this evening and it ran fine, save for one slight stumble when pulling away from a traffic light. Seems the possibilities are:

Air flow meter
Wires (appear to be in excellent condition)
Bad gas (Chevron 93 octane)
Fuel filter
O2 sensor

Any of these make sense?

Will install new chip and replace fuel filter this weekend. I guess I'll keep driving it and see what happens.

Can an 89 be diagnosed via a Bosch Hammer like the later models? I need to get my pre-tech inspection for the Ala PCA DE soon so perhaps I could get the DME scanned for codes...just not sure if this can be done on a pre-964 car.

Thanks,

It's the tail. Needs to be..more, well more ducklike.:D

Mike,
Does it idle great and start easily?

In my limited experience, high rpm (only) missfires have been plug wire related. I don't think any of the other things you mentioned are likely to cause your problem.

I've seen a few reference and speed sensors get "weak" and fail as the temperature got warmer.

Humor me, on the engine, find the 3 connectors just to the left of the heater blower. (You may need to remover the plastic heater hose to get access.) Grab both halves of each connecter and gently pull and wiggle to see if any connectors are loose. Now squeeze and wiggle each of the connectors together.

As anyone who has put SSI's on a 3.2 can tell you, the less restrictive exhaust will cause things to go lean and give you that popping when you lift at high rpms.

Like you suspected, you can't use the hammer on pre-964 911's.

Mark with his ducktail idea may be onto something. :D
-Chris

Chris,

OK, will give it a try. Thanks. Starts and idles fine.

Quack, Quack.;)

are you sure? i thought my bentley said otherwise.

It isn't (adaptive) that I'm aware of. When you turn off the car I think the DME looses power completely. I haven't investigated enough be sure that there is no non-volatile memory in the DME but I'm not aware of any.
Maybe Thomas Owen or Tim Polzin will weigh in here. They've played around with the DME guts more than I have.
-Chris

Two things:

My first thought was plug wires, too. Does your car still have it's original set? My '88 wires look very, very good, but they light up like a sparkler if they get wet near the distributor cap. New ones are next on the list.

I'll also bet the Wong chip will go a long way toward correcting the problem. I also have one in transit from him.

Wavey

'88 Carrera, Dansk Sport w/84MM tip, soon to have Euro premuffler (you know the story), soon to have custom chip.

Forgot to add: you can spritz the plug wires with water in a squirt bottle to check them - try it in the dark, the light show is great if they're bad!

W.

Me, sarcastic? Never!

Have replaced spark plug wires on almost every P-car I've owned. Those and a leaky distributor are the first two items I'd check.

Just my thought.

Does the miss at 6000 occur near or at full throttle/load? If it occurs on light throttle load, and also while idling or near idle, there is a strong possibility you could also have intake air leaks, creating the extreme lean condition which will contribute to ignition misses. Adding the free flowing mufflers like you did will only help to exacerbate the problem. I once had this problem, misses, loud backfires, and found leaks all over the place. The worst was a disconnected hose that was supposed to be connected to a nipple on my throttle body, bringing in too much air at manifold vacuum. At full throttle there was not problem because the pressure inside the manifolds is basically at atmospheric, so the fuel mixture was not thrown off and causing any problems.

On most 3.2s the most common cause of intake air leaks are the intake manifold gaskets. There are 12 of them between the manifolds and the heads, and they shrink with heat and age, causing a varied amount of air leakage at each cylinder. Some may be worse that others, with the worst lightly missing at idle. More than 50% of 3.2 will have this problem if they have never been taken care of. Take a can of starting fluid (ether) and spray them at all potential leak points with the engine running. If you notice a sudden rise in idle, you have a leak. You can try to retighten the manifold bolts to spec, about 16-18 lbs. Don't overtighten as you can warp the manifold flanges.

Thanks for the responses. Car idles fine. The missing primarily occurs when I release the clutch and begin to accelerate in first gear (at low rpm). It will stumble a bit and then accelerates fine. Twice yesterday it stumbled while under hard acceleration (WOT) at high rpm though, hmmm. Hasn't done it since. I wouldn't say that it backfires, it's more of a nice sounding burbling when I lift.

I changed my fuel filter (needed to be changed anyway) and checked every wiring connection I could find, including removing and reattaching all spark plug wires. I plan to replace the wires this weekend just to be sure (records from PO do not indicate that they were ever changed...although they look OK). Cap and rotor were replaced about 8,000 miles ago...plugs replaced about 3000 miles ago. I tried the squirt bottle trick, but it was daytime...no sparks visible. Went for a drive and it ran fine...it may have stumbled very slightly one time but it was minor and I couldn't reproduce it.

Will try to get more seat time in to see if it will happen again. Good idea on the manifold bolts...will snug em up.

Aaah, would you say it stumbles under load then? Isn't that when the ignition "works" the hardest and when bad ignition would reveal itself?
-Chris

Agree with Chris, stumbling under WOT usually relates to an ignition problem. At WOT, cylinder compression is at its highest, the most difficult time for a spark plug to ignite, so high voltage will look for the path of least resistance, and if one exists such as through pinhole leaks or cracks in the plug wires, the spark will jump out of there instead. Weak and dying ignition modules can also contribute a weakening spark condition, however the Bosch modules are usually pretty reliable.

Discovered my full throttle fuel enrichment switch wasn't working (checked for continuity, and got none, regardless of throttle position). Removed it, opened it up and it seems the contacts had warped just enough out of position to prevent contact regardless of cam position. Simply bent them back in place and reinstalled. Could explain the one time I had WOT miss...starved for fuel. But it doesn't explain the low rpm stumble (which I couldn't reproduce today). At WOT, the power delivery is impressive and without a hiccup.

Still planning to replace wires this weekend and install the SW chip.