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Backfiring under acceleration.

S

sniperthx1

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My 87 v10 is having issues.

When the truck is parked, it idles fine, but if I give it quick throttle, it backfires. I can give it gentle throttle and get it to the higher rpms just fine.

Does the same thing while driving, except that it also lurches horribly when it backfires.

It sat not running for the last 4 years, and still had some of the old gas in the tank until today.

Going to check compression tomorrow, and take a look at the ignition components.

Anybody have any suggestions?
 
So many possibilities.........
Check for rotted vacuum lines, especially the vacuum advance if it uses one, dirty pot on the TPS, just bad gas in general.
 
Bad cam lobe?? I would check the easy stuff first though. Fuel filter maybe, and fuel system, spark system, vac system.
 
I'd blame the gas before anything else--I've been regretting using some gas I got free from a car a frirend junked that was only put in the car in February,and it smells like stale beer,and my mowers and tractor didn't like it very much--uts like it wont ignite,its not dirty or watery,it just lacks volatility...I put a bottle of fuel injection cleaner in it and it ran 100% better ,now at least they will START without ether and hopefully it wont fould the carb up...

I would add something to the fuel and run it till its all used up,then see if a few gallonms of fresh gas makes it any better before changing any parts or adjustments,if it ran OK when parked...todays gas goes sour in less than 90 days under the right conditions...
 
The tank had an 1/8 tank of bad gas, so I filled it up with fresh gas after emptying as much of the old gas as possible without dropping the tank.

Going to add some fuel injector cleaner, and do a tune up. Run it for a bit.

In the meantime I'm going to take a look at the cap, rotor, etc.

I wonder if the Spark control module could do it. I didn't put any dielectric grease beneath it when I installed it, so I've likely killed it.
 
It's amazing how bad fuel can cause so much grief. My '90 Blazer sat for ~2 years before I bought it. It had a bad Coolant temp sensor the previous owner couldn't diagnose. With the help of a few guys here I got it running after handing over the cash and drove it home. It ran 'OK' but not like it should.

I did a complete tune on it after getting it home, added a cat (was missing) and it definitely ran better. When i took it to be E-tested it started to run horrible while I was sitting in line to be tested. I ended up pulling out of line and went to the gas station and filled the tank with ~20 gallons of fresh fuel. Then I drove it for about 45 minutes, occasionally ripping on it when there was enough room to do so. The roughness eased up and then went away and by the time I got back to the test station it was purring again. It hasn't done that to me since.

it passed the E-test with good numbers...and I drove it for about 8 months straight. The next E-test I had to do with it the numbers were significantly better than the first test, and the only thing different about the truck is that it had been regularly driven.

Rene
 
No ses light? Last time mine did like you are describing, it was a weak fuel pump, caused the engine to run lean, idled fine, just nothing when ya hit the gas pedal and would backfire and sputter along. If it's been sitting awhile, I would replace the fuel filter as well.
 
I tested my Ignition control module, it failed. So I replaced it.

Now it doesn't backfire under acceleration, but it backfires when I let off the throttle.

G
 
Sounds like you have two halves of a good module. Maybe if you could glue them together............
Naw, then it would probably backfire all the time!

Double check your plug wires. You might be getting some crossfire. It sounds like the plugs are firing when the intake is still open.

When you say backfire, are you talking about back through the intake, or out the exhaust?
If its the exhaust, then it sounds like it is getting air even though the throttle plates are closing and causing it to dump a really lean mix into the system.
 
Not sure whether it's from the exhaust or intake.

And I'm not sure on the timing, need to track down a timing light and check.

Checked the compression earlier. All between 105 and 110.
 
I feel stupid.

I had two plug wires crossed.

Rearrange them, no more backfiring.

At least it isn't backfiring now, I hesitate to call the problem gone until I've tested it a bit more.
 
110 psi compression is too low i like to see at least 135 to 150 but they are fairly equal did you have the throttle open all the way when you did the test and also after you do the test the first time squirt some motor oil in the cylinder and try it again if it goes up considerably then you have a ring sealing problem
 
Hmm, the engine runs perfectly, now that I replaced the plugs, wires, cap, rotor, and the bad ignition control module. No smoke, no problems like that.

And it appears I may need to redo the compression test for the right numbers. I left the throttle closed while cranking.

BUT, I'm not too worried.
 
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