1989 Ranger 4x4 backfiring and stops running at idle PLEASE HELP! - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 05-04-2014
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1989 Ranger 4x4 backfiring and stops running at idle PLEASE HELP!

I've replaced the plug wires, plugs, coil packs, crank position sensor, timing belt, cleaned all grounds and wire connections, added one ground from engine block to frame. Replaced ignition module, fix all vacuum leaks. This truck does have a map sensor.
What happens is it will crank up but will not idle due to backfiring thru intake. Will run at higher rpm smoothly but once rpm drops to idle speed truck will back fire and cut off. PLEASE HELP I'm at wits end about this!
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Old 05-04-2014
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So you believe it is an electrical issue and not a fuel issue?
The IAC(idle air control) valve is how the computer controls the idle.
1,000rpms cold
700rpms warmed up.
The IAC uses a step motor connected to a valve on an air passage inside the intake.
It is, in essence, a controlled vacuum leak.
Step motor allows computer to return/set the valve at the same place each time.
Most engines will die if IAC valve is unplugged while engine is idling, they should drop to 500rpms, lol, but most die.
If IAC Valve was "dead" then most engines would not idle.
It can be removed and tested, just connect it while it is off and turn on the key, you should see/hear the motor and value move.

Does it backfire when engine is cold?
If so then either spark is too advanced or valve timing is wrong.

If it only back fires when warmed up then fuel mix is too lean, hot cylinder is igniting it before spark plug can.

Does it have "power" if you drive it at higher RPMs?
If it lacks power it could be valve timing, lack of fuel or MAP sensor, I assume you have checked the MAP sensor's connector and vacuum line and they are both OK, MAP sensors rarely fail, not never but rarely.
If you unhook a MAP sensor on a running engine it should still run but will develop a lope at idle and have poor acceleration, but would still be drivable.

Any compression numbers?
You would expect 150+ psi on all 4 cylinders
Lower on all 4 would be valve timing issue.


Does the CEL(check engine light) work?
Does it come on with the key and then go off?
If not it may be disconnected or you have a computer problem.
I would try to read the codes

Last edited by RonD; 05-04-2014 at 03:13 PM.
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Old 05-04-2014
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Thanks for the tips! I will check the compression next and the IAV first. When I start the engine it begins loping and when I hit the gas it backfires and stalls out. However 80% of the time I can get the RPM up enough?
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Old 04-30-2017
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sounds like timing issues to me because of the intake backfire. it could also be a very bad intake valve .. you can rule that out with a compression and leak down test.
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