930 backfire

[930 Guy]

On my 78 930. Pulled the fuel relays before a trip out of town. Replaced them a week later upon return. Car warms up and idles for a minute or so before it starts to backfire at the exhaust. Starts with a popping, and then gets louder, very loud. Drove the car a bit and it ran like the timing was out. Filled the tank with good gas and Chevron. Pulled the plugs, not bad, just a bit blackish. Ran just a bit better. car will drive but runs like it's out of time, and wont get full boost. Upon acceleration, car will backfire. Put my had to the exhaust (Borla Qaud outlet) It is a mini explosion out the tail pipe. Pulled the distrib cap, all looks ok inside, checked the vac advance, it moves some. Not sure how far it's gonna move. Sitting in the garage yesterday car reved up fine in neutral.

Help. Send experts. Where do I start looking?

[WydRyd]

Checked for vacuum leaks?

[Rick V]

That'll teach you to take a trip and leave the car at home. It seams odd that the car would perform like this just by sitting.
I recomend appoligizing to the car for leaving it alone and see if you are forgiven.




Sorry I just couldn't pass it up

[David]

I can't see how pulling the relays would affect anything. Do you hear any new exhaust leaks? I take it this is much more popping than the usual decel popping.

[Henry Schmidt]

It sounds like you are not getting power to the warm-up regulator.
The warm-up reg is a fuel pressure regulator that when warm raises the control pressure causing the engine to run leaner when warm. If it is not working the car starts well but then gets very rich causing back fires and dying at idle. If a fuse or wire to this regulator comes lose it will act just like you describe. The same circuit operates the auxiliary air reg. This add to idle speed when cold.
I would look to that circuit.
Good luck.

[David]

Now the Henry mentions it, it does sound like a CIS pressure problem. Is there any popping from the intake?

[930 Guy]

Rick V. I tried to find someone to drive it for me while I was at Road Atlanta, no takers! 125 Shifter, More popping then the dec sounds. I haven't found any exhaust or any vac leaks. No popping on the intake side of things. Thanks Henry, I'll check your tips.

[Paulporsche]

Popping from the exhaust usually indicates a too rich mixture, as others have said. Maybe your setting is off. What happens if you lean it slightly (say in 1/16 turn increments)? If you do not feel comfortable doing this, have someone w/ an analyzer check it.

Just another thought: I know the SCs didn't get one til 80, but maybe the turbos were ahead of them. Do you have an O2 sensor?

[Henry Schmidt]

Originally posted by Paulporsche
....... What happens if you lean it slightly (say in 1/16 turn increments)? ...........

Bad idea, what this would do is mask the symptom and what we want to do is cure the problem.
If it is running too rich, we want to know why and only then do you want to change something.
It is very common and tempting to try and repair a problem by simply adjusting a screw. Sometimes an adjustment is all you need but most often something has changed (failed) and by changing the adjustment the symptom may become nullified but the problem still exists.
Find out what is missing and you'll know what to fix.

[Paulporsche]

Henry,

I don't disagree in principle, which is why I suggested bringing an analyzer to the party as an alternative, especially if someone is uncomfortable w/ doing their own adjustments.

However, you know that sometimes these things need to be adjusted because of changes in ambient temp. This is very common here in this part of the world, although I admit our temp extremes are probably more varied than the poster's.

930 guy,

Do you have a fuel pressure gauge so you can check your control pressures?

BTW, does anybody know if these cars had O2 sensors or did they come in '80 as w/ the SCs?

[930 Guy]

I've ordered a new large vac line to the aux air valve to be certain it doesn't leak. I'll follow up on Henrys advise and check for power at those circuits. I have been blowing a fuse to the blinkers on a reg basis. Does anyone know if this is on the same circuit?

[930 Guy]

Update, this am I put on the to-short vac line to aux valve. Car acted the same (still popping) with the addition of uneven idle up and down. Pulled the plug to Aux valve, I have power to it. Pulled plug to WUR, power as well. Replaced power plugs and started. Ran fine , nearly 95 % of the power it always had. Seems to be ok. I have less popping of decel. I don't yet have gauges to do CIS tests. Does this mean my WUR is bad?

[Paulporsche]

Most likely culprit is an out of spec or bad WUR. If you search WUR you will see they usually go out of spec but can be reset. By checking your control pressures you will be able to tell.

[930 Guy]

Thanks everyone. Running much better. I advanced the timing a couple degrees. (the cams were clocked wrong when rebuilt so I can't use the timing marks) Once I purchase CIS gauges, I will test all the pressures and adjust accordingly. b