I have some backfiring going on and I'm running out of ideas of what can be the cause. The backfiring gets more outspoken in higher revs. Also, if idling when I hit the gas, I first wants to stall before I will actually go up in revs.

I've done new distributor, rotor, cap, spark plugs, spark leads.
Verified firing order multiple times.
Checked all intake gaskets and checked cylinder compression.

1: 130psi
2: 130psi
3: 125psi
4: 125psi
5: 125psi
6: 140psi
7: 135psi
8: 140psi

What next to test?

 

What year/engine etc etc. I am curious about your compression numbers. I have an 84 with the L83 in it and my compression numbers were 160, 160, 160, 155, 150, 145, 140, 140. seems like you could have some low numbers or your motor is just way different than mine. Either way there is not enough information here to diagnose anything... Is there smoking? when does it smoke? fuel mileage? I'll be watching this thread.

 

O2 sensor....s

 

85 corvette with L98 tuned port injection.
No smoking. Fuel mileage is pretty good.

 

Can anyone tell me if the compressions would or should be that far apart between the L98 and L83?

 

Service manual just says it has to be above 90psi. These readings were also done with a cold engine, so I do expect better readings at temperature. I didn't let it warm up as the issue is there both cold and at temp.

 

Sounds like a plug wire out of position or retarded timing. Have you moved the distributor? If not, don`t. Is number 1 plug wire in the exact same position in the distributor as before? Pull #1 cylinder spark plug. Get #1 in fireing position and the timing mark on 0. Pull dist. cap. Rotor cap should point directly to the #1 plug wire. Your compression numbers have verified that you don`t have a bad valve. If you moved the dist., try moving it in the counter clockwise position while looking at it from the top. 18436572 is the fireing order.

 

fuel pressure regulater good luck!

 

make or buy a piston stop. and use it on cyl1 to verify that your timing mark is correct.

my 85 was back firing like crazy and i had 2 things at once: dizzy off by 180 defrees and rocker arma too tight so my intake valves were not fully closing.


u have spark going off while tour intake valve is open.

 

Have you checked to see that your EGR is working correctly? This is probably where I would start looking. When my EGR failed I was shocked at how well my car ran after it was fixed. My EGR solenoid failed open and caused my emissions numbers to go bonkers. The solenoid was not available at the time I had the problem so I used a similar one with identical functions to get going again.

How about your vacuum hoses? My car was running rough and popping due to a small crack in the vacuum hose going to the cruise control system.

Is the A.I.R. system working properly or could one of the check valves gone bad?

Years ago I ordered a kit from Mid-America that was called a:

"Emissions Sensor Test Harness Tools Combo Kit 1985-1989"
Part #: 601074

It goes for about $100 and will let you verify that all the primary sensors are set properly or working properly. This kit is an invaluable part of my Corvette tool kit. I have used these tools to get several Corvettes on the road again without breaking the bank to do it.

 

did the open egr cause backfiring?

or a real bad/rough idle?

 

Verified this. Cil #1 TDC and rotor points to #1. Played around a little with the timing, but it has backfire within the entire range.

Btw, the backfire is not constant. It does it especially on a sudden big increase in throttle or go full throttle. If you build up the rpm, the backfire is not there.

 

EGR is replaced when I did all the intake gaskets.
I believe the vacuum is fine as the cruise control is still working as expected.

 

Checked the fuel pressure at the fuel rail, that looked all correct. However, it smells. You better change clothing after driving it.

 

Pull the vacuum line off your fpr and smell for fuel.

 

There is a light smell of fuel. It hasn't been running since yesterday.
So I guess FPR?

 

Did you check the cat to see if the cat isn't dead and giving the engine a ton of backpressure?

 

How do I check that?

 

Put it up on a lift and take the cat off. It is located slightly behind the transmission in your car I believe. Make sure the pipes and everything is cool and not hot if you ran the engine just before doing the below.

If I remember correctly your 85 has the same exhaust set up as my 84. This would be a front y pipe that takes dual 2.5" pipe and brings it into a single 3" pipe. The cat is literally right after it. Then there is an another y pipe after that that breaks out the single 3" pipe to dual 2.25" (not 2.5") pipes that then go to each muffler. You first have to make sure that the bolts holding the cat to the 3" pipe on each side is not old as hell and potentially weakened to the point where trying to take it out would break it in half. Heat solves that issue but you need to know what you are doing, get a friend who knows how to work with heat if you don't.

When you take the cat off you will inspect it. Look at the side that is closest to the engine. Does it look melted? Sometimes unburnt fuel flows through the exhaust system and lands inside the cat where it starts to burn due to the sheer heat inside the cat. Check to see if the material inside the cat isn't breaking or "flaking". Basically, does the cat look normal or does it look like it is falling apart or a solid mass inside? Shake the cat, does anything jingle inside?

Sometimes you can smell the exhaust as it comes out of the mufflers and it smells like rotten eggs, this is a sign of a dead cat. And when we say "dead" it doesn't always mean it is non-working. Sometimes it is only marginally working to a small degree but is still causing a ton of resistance to the air flow of the exhaust system.

 

So I checked the FPR, looked clean, no rips. I re-positioned it, so the moving valve is in line with the feed. Made no difference.

Checked fuel pressure again:
Without vacuum connected to FPR: 37psi
With vacuum connected to FPR: 28psi
However, when I blip the throttle and there is a backfire, the fuel pressure spikes to 37psi.
When I gently increase throttle it stays stable around 28psi.