3.0 miss and backfire

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ctwrench

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Ok, first off, I've read all the posts that seem related and I'm still not there. To begin, my 92 had 209k on it when I talked to the original owner and found he had never done a 60k. Straight to the shop for a timing belt, plugs and wires. After picking up the car, I got 2 blocks down the street and while sitting at a light, it died. Fire it up and continue on to work wondering why my car is missing and backfiring. Return the car to the shop and they pull everything apart to check the timing. Everything is ok. New IACM and crank sensor. Still no help. They say I have crud in the tank. I pulled the tank, cleaned it, new screen and filter, flushed the injectors and cleaned the butterflies. No dice. I took it back to the shop where they tore it apart again with the same conclusion. The tell me I'm supposed to have 190-200 lbs of compression but #2 only has 130. Valve job is $1800 to r/r head + whatever the machine shop is going to charge. I figure I'll try a second opinion and hopefully cheaper solution. I pick up the car and it is running like absolute junk. Enter new shop. They find that the first shop left a sensor unplugged, but cannot find anything else. Wife suggests checking the ignition module. It failed dwell and threshold test at the Zone. New module didn't help. I figure what the heck and throw a new coil pack in. No help there.

The particulars: The car misses and backfires on take-off. The car misses at steady speed. At highway speed, pressing the gas down to speed up causes more missing and often backfires. I'm going to clean the MAF tomorrow night, but any other ideas would be greatly appreciated.
 

Kens1992mtxSHO

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Ok, first off, I've read all the posts that seem related and I'm still not there. To begin, my 92 had 209k on it when I talked to the original owner and found he had never done a 60k. Straight to the shop for a timing belt, plugs and wires. After picking up the car, I got 2 blocks down the street and while sitting at a light, it died. Fire it up and continue on to work wondering why my car is missing and backfiring. Return the car to the shop and they pull everything apart to check the timing. Everything is ok. New IACM and crank sensor. Still no help. They say I have crud in the tank. I pulled the tank, cleaned it, new screen and filter, flushed the injectors and cleaned the butterflies. No dice. I took it back to the shop where they tore it apart again with the same conclusion. The tell me I'm supposed to have 190-200 lbs of compression but #2 only has 130. Valve job is $1800 to r/r head + whatever the machine shop is going to charge. I figure I'll try a second opinion and hopefully cheaper solution. I pick up the car and it is running like absolute junk. Enter new shop. They find that the first shop left a sensor unplugged, but cannot find anything else. Wife suggests checking the ignition module. It failed dwell and threshold test at the Zone. New module didn't help. I figure what the heck and throw a new coil pack in. No help there.

The particulars: The car misses and backfires on take-off. The car misses at steady speed. At highway speed, pressing the gas down to speed up causes more missing and often backfires. I'm going to clean the MAF tomorrow night, but any other ideas would be greatly appreciated.

Check to make sure the plugs are on in the right order...

Engine

(Firewall)
123
456
(Front of Car)

Coil Pack

51
62
43
 
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johndallara

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ouch. make sure they hooked up the coil correctly. several posts on this. the #5/6 are reversed, easy to mix up when re-connecting the plug wires.

not sure about the compression difference, but the upper 60k and stuff you mention should not have affected a bad cylinder -- was the car running well before the shop got it ?

cleaning MAF good idea ? don't know if it will help.

JD
 

ctwrench

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Not feeling well today, so haven't done anything to check out the firing order or MAF. The car had a slight miss before taking it to the shop, but I figured it was old plugs/wires. It was nowhere near as bad as now. The second shop did a compression test and came up with 130 - 150 lbs per cylinder on all 6. I don't think it is a compression issue.
 

Kens1992mtxSHO

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Not always, if it's just dirty. I know mine didn't when I had MAF problems.
 

Mad Celt

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Put it in 4th gear at about 20mph while going up a hil...mat it. pull over in a minute or two and see which Cat is glowing RED HOT. That will tell you what bank of cylinders to start looking at :)






Don't really try this....you could really screw your shit up. Melting CV boots is just a start...burning the pig to the ground is the other end of the spectrum....
 

Storm-Chaser

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If the plug wiring order is correct, I'd next question whether the timing was set correctly. It's possible that the belt was mis-aligned (off by a tooth), jumped (tensioner weak or improperly tensioned), or possibly crank-cancer (balancer woodruf key). While the woodruf key is the least likely of the three, I would suggest checking the key/keyway if you pull the timing cover to check the timing.


Backfiring is almost always a timing problem or plug wires in the wrong order.


:burnout:
 

93MTXSHO_STUD

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If ole sparky doesn't fix the problem id look at the DIS (though I agree with everyone else. Def sounds like a spark plug wire problem). You said you replaced it, but did you make sure the cable was properly grounded? When I did not properly ground the braided ground, I had similar symptoms. I believe I had a CEL though. Just a thought. If the person who took off your manifold to change the plugs grounded that braided wire to a different spot, there could be high resistance path to ground causing a problem (painted manifold). You live in Houston? I live in south Houston. If you still need help send me a PM and I can help you out.
 

ctwrench

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Ok, checked the firing order. Everything seems to right there. I cleaned the MAF. Still the same problem. Looks like I'm tearing it apart over the weekend myself to check things out. ** I don't have time for this!!! ** The whole reason for having a shop do it in the first place. What a pain.
 

K-Dawg

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The whole reason for having a shop do it in the first place. What a pain.

That's the reason I DON'T have a shop do things on my car.:salute:

I just noticed that you are in Houston. You have one of the best SHO mechanics around right there.
 
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Storm-Chaser

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Why aren't you going back to the shop? Why are you doing work, you've already paid them to do? The fact that they failed to properly connect all the sensors and miss this during two subsequent checks, makes all their work suspect.

My money is on the belt being off one tooth at the balancer (crank pulley).

And if it is, take pictures, go back to the shop, and ask for your money (for the labor) back.


Ok, checked the firing order. Everything seems to right there. I cleaned the MAF. Still the same problem. Looks like I'm tearing it apart over the weekend myself to check things out. ** I don't have time for this!!! ** The whole reason for having a shop do it in the first place. What a pain.
 

dtm1

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Misfire

Look at the connector to the crank sensor by the middle left of your intake manifold- I had a similer problem- make sure its in its keeper
 

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