BesideTheBox

My apologies for the video. It is long.
I combined audio from a recording device 3 feet behind the car with an in-car video of the oil and tachometer gauges.

Here are some time indices for various key points.

• engine start: 0:00
• flapping noise start: 1:23
• engine rev causes CEL: 2:43
• erratic popping start: 3:25
• injector 1 removal: 4:40
• injector 2 removal: 5:06
• injector 3 removal: 5:40
• injector 4 removal: 6:31
• injector 5 removal: 6:54
• injector 6 removal: 7:30

The flapping noise is new. The only thing that is different is that I had pulled off the mufflers, shook em, tapped on 'em and put 'em back. The right muffler band was originally tighter than it is now. Maybe it 'needs' to be tight to stop the muffler from flapping?

jcochran1

Whoa that sounds worse now, my car doesn't sound like that and my noise is only audible from outside if you're down by the muffler. Good luck!

IainM

I don't hear the popping sound.

I hear the flapping sound which to my ear lags the engine revs and definitely the tach. Makes me think it's something banging around in the exhaust. But then the roughness of the engine doesn't show in the tach. Mine drops if the idle misses a beat.

This could also be a vacuum leak somewhere. It's the engine smarts outsmarting itself.

IXLR8

I've been thinking and based on what I've read on various forums & threads what about a hydraulic valve lifter. Not a collapsed lifter which would be obvious, but one that is binding.

Brad, you have a PM with a link.

vincer77

Any updates?

BesideTheBox

Thank you for asking. Yes.

Executive summary:

Shop diagnosed light misfires at idle due to a lean running condition. Suggest I install a new mass air sensor. The finding of a voltage supply CEL code prompted me to clean numerous grounding points which appears to have remedied my initial P0306 CEL code and associated poor running during engine warmup.

Recap and details:

A month ago, I changed my motor mounts. The next day, the car began running rough as it was warming up. Sounded fine initially, then went through a rough patch, which produced a CEL, and then ran reasonably well when more fully warmed up. I checked the CEL four or five times and it was always P0306. At that time, I started a thread about diagnosing a P0306 CEL code.

To address the P0306, I did the following:

Fuel injectors - had them cleaned
Spark plugs - replaced
ISV - cleaned w/ carb cleaner
MAF - cleaned w/ CRC MAF cleaner
V-belts - replaced all three
plug wires - yes, original - ohm checked twice
caps & rotors - in good condition, only 4K miles on them

None of this made any difference.

So then I started this post because of the erratic popping that I was hearing out the exhaust. I wondered if they might be related.

For me, things got a little crazy after I pulled the mufflers and concluded that the erratic popping did not appear to be coming from the mufflers. I won't be able to completely eliminate this until I am able to try a set of known good mufflers on the car.

So last Friday, I went to a local independent Porsche specialist. On the way there, the car was running terribly. Hesitating and bucking while driving down the freeway, and the CEL came on. They checked the CEL and it was the "voltage supply" code (P1600, I believe). They hooked up their scope and could see misfires at idle, varying between cylinders. They hooked up their durametric and could see that the DME was near its limits to compensate for a lean running condition. They concluded that the misfires were not ignition related, but due to the lean condition.

Since I had the voltage supply code, the technician suggested that I clean the power take-off for the DME on the positive battery terminal. While I was at it, I also cleaned the battery ground strap where it attaches to the body.

Friday evening, my wife and I drove the car two trips of 25 miles. The car idled and ran better on each trip for the first 20 miles. Then bucking and hesitating started, but we got to where we needed to go.

Over the weekend, I cleaned grounding points I, II, IV, X, XIV, and XXI. Click here to read about it (post #4).

I still find it hard to believe, but the odd warmup issue that was causing my P0306 CEL is now gone. This appears to be due to cleaning the grounding points. I have driven the car 200 plus miles since Monday and no bucking, hesitation, or CEL.

I still have light misfires at idle. The shop's suggestion is to replace the mass air sensor. I have one on order, should be here Friday.

Another possibility is that I have a vacuum leak somewhere, but I don't seem to have any other symptoms of a vacuum leak - such as hesitation on light throttle.

As always, I appreciate your input. Let me know what you think.

IainM

Doesn't surprise me at all but then I lived under the rule of Lord Lucas, Prince of Darkness. A car is a very hostile environment for electronics and connectors are always a weak spot. Not too good for rubber vacuum tubes either.

Did you check grounding point V, motronic under the drivers seat? Any corrosion there wouldn't be good.

Good opportunity to grease the seat rails :-)

Mike J

Great news, and hopefully the car stays running solid, and you can clear the idle misfires! Keep us posted!

Cheers,

Mike

IXLR8

Coincidence?

Was the motor lowered or raised in the process? And if so, is there any chance that a component was shifted out of place causing an intake leak?

BesideTheBox

I owned a '64 Rootes Group Sunbeam Alpine. 12 Volt, positive ground... The 993s are getting to be that age where more grounding issues will come up.

I redid the motronic ground under the drivers seat. I mis-understood the grounding point table, so I didn't list that point. I confirmed that both points under the left seat can be done without removing the seat. Simply move the seat all the way forward and back.

Grease the seat rails - Heh. how about an opportunity to clean excess grease off of places that it just doesn't need to be? That grease gets everywhere.

Still plugging away at it. At least the rough running, hesitation, bucking, and CEL are staying away.

Yes, and yes. I raised it so I could extract the old motor mount around the AC unit. I looked but was not able to identify anything.

NYC993

I saw issue similar to yours on a Honda forum. A guy had 100k honda with misfires that were cured after he cleaned the ground point, which was in really bad shape.

I'm no expert at car by any stretch of imagination, but is misfire caused in the end by the electric current not flowing to the spark plugs properly through the body of the car and through the engine? Could it be then that replacing the engine mounts messed with grounding of the engine and thus grounding of the spark plugs? Am I totally off here?

BesideTheBox

NYC993 - No ideas are 'off'. The recent rough running, hesitation, bucking, and CEL appear to have been fixed by cleaning grounds. If a poor ground is causing inability to complete a spark, then ground point XI - Ground Strap (Starter, Transmission, Engine, Body) would be the likely suspect.

I previously attempted to get at this ground point by going through the right rear wheel well with no luck. Tomorrow I will put the car up on jack stands and see if I have better luck getting at it from below.

Good thing you suggested this. It was suggested to me previously, I checked into it, ran into an obstacle, and then lost track of it.

RS man

Wild guess, I was reading Wayne Dempsey's book for bedtime reading yesterday and it describes a Phat-Phat-Phat sound from older 2.7 engines - pulled head stud. How common is something like a broken stud (rather than pulled) happening in a 993? Perhaps you should e-mail Steve Weiner and ask his opinion regarding this abnormal sound - someone like him should have an incling.

BesideTheBox

Update on a few things I have tried.

Mass Air Sensor MAF
Last Friday, I installed a brand new mass air sensor. The computer was relearning idle, so the engine stalled several times during the 30 mile drive. When I returned home, the idle was pretty good and the erratic popping was all but eliminated. I heard an occasional pop, but it was a very light pop - barely audible. The next night, we took the car out and when we returned, the erratic popping at idle was back 100%.

Leakage Test
Did the vacuum leakage test prescribed in the 993 workshop manual. This procedure has been posted numerous times by epj993 and can be found here: https://rennlist.com/forums/993-foru...testing.html#3
The test started at 0.78 bar and there was no drop in vacuum over 3 minutes.

Engine Ground Strap
Without a lift, this is kind of a pain to do. Pulled the strap, cleaned it. Cleaned the mating surfaces around the posts. Reinstalled. No improvement in the erratic idle.

Single Distributor
I mentioned this before. And then I observed differently.
Today, the erratic popping at idle only occurred when both distributors were connected. There is no popping when either distributor was used solo.

This has to mean something!

DME fried?

I've checked the distributor timing and it is correct. This data would suggest otherwise. The independent I saw over a week ago said that the two sparks from the dual distributor do not actually occur at the same time. I imagine that this job was handled by the ignition control module that geolab previously suggested that I replace. In the OBDII cars, this component (or function) is integrated into the DME.

One thing that I can do is perform a leakdown test to verify that there is not a valve issue.

When I was looking over the receipts from the top end rebuild that happened at 44K miles, I figured out that they only did the exhaust valve guides. Should they also have done the intake valve guides at that time? Or is it pretty common to just do the exhaust side?

Feedback and comments are appreciated.

techman1

Not sure how this relates to the distributor observation, but..

Have you checked all exhaust tubing for leaks? I recall popping is only on one side. Maybe try to plug one exhaust tip, blow air in the other with a air hose/vacuum, and spray soapy water on the exhaust.