help 911sc starts but backfires when given any gas

jendon · 06:19 PM

please help my mechanic can't seem to get it straight.Ive replaced my wires
and put in new plugs, it will start but idle very irregularly then when you give it gas at rest in the garage it backfires like crazy starts to stall. I cant even get it out of the garage for 15 minutes. as soon as I try to back it up it dosnt even have the power to back up without popping and then usually stalling...
help I have owned this car for 4 years and never had any problems
what so ever I'm stumped and apparently so is my mechanic
if anybody has any answers please help....

Thanks,

D Smith

JP911 · JP911's Garage

I would start simple and double-check that you have the wires all run correctly to both the distributor and spark plugs. I know that the distributor rotation on an SC and turbo is opposite that of the 911s before and after. Also, check the wiring to the coil, including the grounds.

konish · 10-04-2004, 03:35 PM

D,
After 15 minutes does the car run better? Sounds like you are running lean. What year SC? If its an 80 or later, your lamda computer realy may have gone bad causing you new problem. If its an earlier car, I can't help as to why it has started all of the sudden. First, you need to stop throwing money at it and diagnose the problem. Get a CIS fuel pressure gauge from JC Whitney...I think they are like $40. Test your control and system pressure to see if they are in spec. You might also want to check and see if your temp sensor on the right hand timing chain cover is grounding proerly when cold...again only applies to Lamda equipped cars. This temp switch will enrichen your mixture when your engine is cold. Also, install a pop off valve...backfires have a funny way of cracking air boxes. In fact, if your symptoms started after a backfire, I would almost bet that the box is laready cracked...

R/
Dustin

john walker's workshop · 03:38 PM

please modify your signature to include the year of your car. 80-83 require the frequency valve to be working. the domelight fuse being blown, or a bad relay for the oxygen sensor computer will cause a poor run. the fuse powers the relay which powers the oxy sensor computer, which works the frequency valve, which makes the engine run properly. once that is determined to be in order, unplug the oxy sensor and set the CO to 3.5% with a hot engine, and see if that helps cold running. the switch on the right chain cover needs to be grounded when cold, open w/hot.
78-79, same thing on the CO setting, but no oxy system to worry about. if it's still bad, the warmup regulator may be causing the engine to run way too lean thru the warmup period. a pressure test showing 1.0-1.4bar cold control pressure would be in the normal range. much higher, and a lean run condition w/cold can happen. search WUR or warmup regulator for lots of info on making it adjustable. don't mess with it until you do the first things i recommended, or you may have to undo it. some engines had a thermo-valve that kept the WUR from getting vacuum for a minute or so after a cold start, so the engine could run richer and not spit and pop. follow the vacuum hoses to see if there is one. again, make sure all these things work before messing with the WUR for nothing.

MotoSook · 10-04-2004, 03:37 PM

Is this a new problem? Or did this develop after some work to the engine? Like JP said, check the firing order. It's easy to check, the order should be on the sticker on the right of the latch.

Also, (you didn't say what year SC) check the vacuum advance.

EDIT: and what John said

konish · 10-04-2004, 03:52 PM

D,
After 15 minutes does the car run better? Sounds like you are running lean. What year SC? If its an 80 or later, your lamda computer realy may have gone bad causing you new problem. If its an earlier car, I can't help as to why it has started all of the sudden. First, you need to stop throwing money at it and diagnose the problem. Get a CIS fuel pressure gauge from JC Whitney...I think they are like $40. Test your control and system pressure to see if they are in spec. You might also want to check and see if your temp sensor on the right hand timing chain cover is grounding proerly when cold...again only applies to Lamda equipped cars. This temp switch will enrichen your mixture when your engine is cold. Also, install a pop off valve...backfires have a funny way of cracking air boxes. In fact, if your symptoms started after a backfire, I would almost bet that the box is laready cracked...

R/
Dustin

EWPurdy · 10-04-2004, 04:15 PM

Cracked airbox?

As mentioned above, if you don't already have a pop-off valve:
a) your airbox is either blown/cracked
or
b) your airbox will be blown/cracked in very short order

KobaltBlau · 04:38 PM

Mine does this too but only on occasion, usually on cold/wet mornings in my estimation.

same thing, pops and stumbles so much I can hardly back up if at all. If I didn't have a pop off valve I'd have a blown airbox. After driving 10 minutes, it's all clear and usually doesn't happen again for a month or 6 weeks. I should probably go through John's procedure, mine's an 81 SC.

Can I short-cut any of the steps since mine starts just great on 98% of the cold starts? Seems like my mixture is fine, and certainly the firing order. flaky Lambda relay sounds like a likely candidate ... ?

edit: mine has done this about 4 times in the last 5 months, and I drive it a few times a week normally. I have hardly touched the car in terms of engine mechanicals/electricals during this time, so I don't think I wired something wrong or anything.

EWPurdy · 04:45 PM

Andy,
Testing the pressures with a CIS tester is an easy way to rule out stuff like the Warm Up Regulator which can be a bad actor in some of these cases. Or you can go my route.....

konish · 10-04-2004, 04:50 PM

Andy,
Not sure why you would want to short-cut any of the procedures mentioned. If you can live with your cars cold wetaher behavior then I wouldn't do any of it. However if you want to iron this out, then you really need to do the proper diagnostic. Bascially the tests are go/no go and the a setting of pressures...nothing really involved enough to short-cut. I might think that your car has an ignition problem vs a fuel problem due to the correlation with cold/wet starting and inconsistent manifestation of the problem.

R/
Dustin

jendon · 10-04-2004, 06:51 PM

my car is an 83sc
I only decided to put in new wires when i went by my mechanics and told him of my problem
he tinkered with it and wrestled with one of the plug wires the first plug wire on the drivers side and said the boot was wearing out and i should probably replace the wires soon, he seemed to take it off and push it back on it seemed to greatly improve
I drove it home and then had to garage while out of town. I came back and the problem was back thats when I bought new plug wires and put them on still much the same problem its currently back at his shop with much head scratching on his part. any more ideas once I've clarified the year of the sc would be great

thanks
D smith

sammyg2 · 10-04-2004, 07:38 PM

i'm thinking major vacuum leak. Check the rubber boot that goes from the air sensor to the throttle valve. Sometimes when they backfire the boot pops off.

Oracle · 10-04-2004, 07:53 PM

Quote:

Originally posted by jendon
please help my mechanic can't seem to get it straight.Ive replaced my wires
and put in new plugs, it will start but idle very irregularly then when you give it gas at rest in the garage it backfires like crazy starts to stall. I cant even get it out of the garage for 15 minutes. as soon as I try to back it up it dosnt even have the power to back up without popping and then usually stalling...
help I have owned this car for 4 years and never had any problems
what so ever I'm stumped and apparently so is my mechanic
if anybody has any answers please help....

Thanks,

D Smith

My car had the exact symptoms. Veredict?? Thanks to 25 years of water condensation there was rust in the fuel tank that jammed the piston thingy in the fuel distributor. The mechanic blew it out with compressed air then soaked it and cleaned it as much as possible... Actually he took out the plasitic screens in the distributor cause they were all clogged.
Also I bought a fuel tank, and since then the car is running like never before...

The fuel filter will not stop the rust.

-J

KobaltBlau · 10-04-2004, 07:56 PM

J, thanks for contributing. I'm certain that isn't my problem, but it could be his; I think there are several potentials here...

Oracle · 10-04-2004, 08:20 PM

Well I really hope he doesn't have the same problem I did cause it was expen$$ive. But the same happened to me, I drove the car and everything was Ok then I was out for a week or two and when I came back and drove the car it started to run bad but it got worse cause at the end the car basically died on the driveway and had to be towed to the mechanic... Besides all that backfiring caused the pop-off valve to go off many,many times, actually it burnt my arm when flames came out the valve (one big explosion) while I was messing with the idle screw.

Kids don't try this at home.. ;-)
-J

VaSteve · 10-06-2004, 04:48 PM

Quote:

Originally posted by john walker's workshop
please modify your signature to include the year of your car. 80-83 require the frequency valve to be working. the domelight fuse being blown, or a bad relay for the oxygen sensor computer will cause a poor run. the fuse powers the relay which powers the oxy sensor computer, which works the frequency valve, which makes the engine run properly. once that is determined to be in order, unplug the oxy sensor and set the CO to 3.5% with a hot engine, and see if that helps cold running. the switch on the right

JW, is this correct for an 80-83 SC? In the Bentley it states .4 - .8% with the O2 sensor disconnected for an 80-83. Before I foul mine up, I want to make sure.

ryan j · 10-06-2004, 05:05 PM

Here's my experience, might be similar, might not.

Hard to start, need to give it gas at times to get started. Idles rough, begins to smooth out as RPMs increase, then occasionally backfires and stumbles. won't go over 5k.

found a weak coil after car refused to start. replaced it and it's all good.

Rick Cabell · 10-06-2004, 06:24 PM

You should probably start resolving your problem by taking your 911 to a mechanic who knows what he's doing. Your wrench may be a great guy and all, but if he doesn't know how CIS injection works, you car could sit there for months.

The problem isn't -- and wasn't -- the ignition wires.

Thack · Thack's Garage

Jenden,
Go to the auto parts store and get some starting fluid. When you spray it in the air breather it should smooth out or increase RPMs. If this happens you are running lean. Then take your car to someone who has CIS test gauge equipment and test the pressures. Be careful with the starting fluid please.

Mike

emcon5 · emcon5's Garage

Quote:

Originally posted by VaSteve
JW, is this correct for an 80-83 SC? In the Bentley it states .4 - .8% with the O2 sensor disconnected for an 80-83. Before I foul mine up, I want to make sure.

They are both correct. The Bentley/factory figure is to make it run clean. JW's figure is to make it run well.

jendon, before you spend any more money, do some free troubleshooting:

Make sure your frequency valve is working. Depending on how loud your exhaust is, you may hear it buzzing, but you should feel it vibrating if you touch it when the engine is running. There is a good explination on locating it here:
Can you feel a duty cycle valve running?

If it is working, then start looking at testing fuel pressures and checking for vac leaks.

If it is not vibrating, do your dome lights work? The Lamda system gets power from the dome light fuse. If your dome light works, reach under the passenger seat, and locate the power relay next to the Lamda system brain. Unplug the relay and start the car. If it runs the same, the relay is probably your problem.

Otherwise, scroll back up and follow John W's instructiuons.

Tom