944 Rough Idle & Backfire ???

Porsche37 · 12-31-2014, 09:29 AM

Searching for the elusive reason why my 85.5 N/A is running very bad...

The end of a three year project and the engine was removed and put back in, all new vacuum lines, plugs, injectors and everything in between replaced new. Engine ran great until now... Out of the blue, she just started to run bad. One mechanic suggested that since I have only been idling it and moving it in and out of the garage for over a year that maybe one of the rings have tightened and causing loss of pressure in the cylinder. He said to fix it just take it out on the road and run it a while nice and hard. I never heard that before. I was willing to do this just to see if it works, but in the final stages of painting restoration. I don't even have a windshield now... Any ideas??? I am totally stumped.

GTR34 · GTR34's Garage

I'd pull the belt covers and make extra sure the marks are still lined up. Just to make sure the belt didn't for some reason slip a tooth - I doubt it, but you never know. That's where I'd start.

Bradical · 01-03-2015, 03:53 PM

If your mechanic is right then you can prove it with a compression test. What he is talking about is the piston rings needing to wear in against the cylinder walls and seat properly, presumably. Buy or rent a compressor tester and check each cylinder. If you have low compression on one or more he's probably right.

Jfrahm · 01-04-2015, 04:58 AM

Is it really a backfire or is it chuffing out the intake? Backfires are loud bangs of exploding fuel/air in the tailpipe. A misfiring cylinder can deliver the fuel/air but it'd be hard to get one without enough heat in the exhaust to ignite.

I have not heard of what your mechanic suggests exactly, maybe poor valve seal or ring seal casing poor starting due to low compression but not something like this.

In your case I'd suspect a slipped timing belt (as suggested, check this first!) or an ignition problem, possibly DME damage from jumpstarting. Maybe S/R sensor wiring trouble.

-Joel.

Porsche37 · 01-04-2015, 07:30 AM

Hey, this gives me a start! Thanks for the thoughts on getting this back to running condition. She has regressed to a point that she will not start. Turning over, but now sounds like only one cylinder is firing. Yes, there was a backfire, but behind the cat I have the exhaust removed prepping for the paint booth. Timing belt it is then, I will see what gives... Had this redone with a new turbo water pump in the beginning of the overhaul.

guru944 · guru944's Garage

Porsche 37,
Check Fuel pressure especially regulator. That may have gone bad. Mine did that last year and had to replace it/upgraded.
later...

mattdavis11 · 01-04-2015, 12:38 PM

Sounds like bad gas to me. Throw some fresh fuel in it.

nynor · 01-04-2015, 05:19 PM

Quote:

Originally Posted by mattdavis11

Sounds like bad gas to me. Throw some fresh fuel in it.

this.

Porsche37 · 01-11-2015, 11:31 AM

Read my mind, new gas added with some gas additive. Charged the battery and cleaned the plugs. Two plugs were fouled bad and the other two at a real close second... Started but fired it up only a few minutes. Sounded like before, one cylinder or two not firing.
Replaced fuel regulator and damper 3 years ago at start of overhaul with new ones. External fuel pressure gauge reads around a constant 30+ and keeps the pressure. New fuel pump and internal and external filter... Ask me what I did not replace.
I plan on pulling the timing belt cover, had the air box out and checked the maf and tb. Thanks for the info, help is appreciated.

mytrplseven · mytrplseven's Garage

If you lost a vacuum line to the FPR, it will act like it failed. Check all your vacuum lines, especially under the intake manifold where the Y fittings are hidden and the big ones underneath the J boot.