Backfire through intake on startup

[B2Mann]

Quick question all. I've got a 91 ranger with the 4.0 and every so often when I start it up itll backfire through the intake. Not that big of a deal, truck runs great otherwise. Just wondering if maybe there was an issue. Thanks

 

[B2Mann]

Anyone want to throw there 2 cents at this?

 

[marcuspur]

your timming may be off by a little bit mine does it when i rev it up cuz i havent set my timming yet

 

[n_grimm]

Don't see it as much with fuel injection but with the old carbed rigs, they would pop back thru the carb if they weren't getting enough gas. I wonder if the iac was getting stuck open if it would cause that. There's my 2 cents.

 

[B2Mann]

I dont think there is a way to set base timing on a 4.0, since its distributorless. I can understand the carb aspect, with it backfiring through the carb from there not being enough advance in the spark, or too much fuel. My truck has been idleing higher than it should 1100-1200rpm warm, and im wondering if the IAC isnt closing all the way. Was reading other high idle forums and unplugging the IAC valve does drop the trucks idle to 850-900rpm. No vac leak. I wonder if the backfire issue is fuel related.

 

[Big Jim M]

For sure I'd be starting with the iac. A backfire can be caused by several completely different problems. Since a LEAN mixture burns faster than a NORMAL mixture an iac setting that would be for RUNNING instead of STARTING could cause a backfire.
Also a timing setting of too ADVANCED could cause that problem.. Then the WORSE would be a leaking intake valve allowing the contained fire to leak back into the intake at startup.. Which would also be noticed in normal driving.
Big JIm

 

[B2Mann]

It seems to run awesome while driving normally, just a high idle when at a complete stop. The truck hasnt backfired in the past week. I changed the housing on the IAC with another IAC housing that has smaller openings. It helped a little, but still idles high. Isn't the IAC computer controlled?? I also noticed when removing the PCV valve and plugging it, the idle drops down to 800-850rpm. All other sensors are working correctly.

 

[feellnfroggy]

On the older motors of that year, you can set the cam/crank timing. When your done setting the cam crank timing you reinstall the spout plug and the computer will advance the ignition timing automatically. On the smaller 2.3 with belts its easier to adjust the cam/crank timing. but on a 4.0 I think is chain and a little harder to mess up, but still possible. Without the spout plug it runs at ten degrees default. It sounds like your timing is off ca/crank timing. The intake valve is left open a tiny bit when the firing cycle begins allowing the combustion process to bacflow through the intake.

 

[B2Mann]

Thanks for the imput. Any tips on how to adjust cam/crank timing?? Distributorless ignition is a new thing for me, so learning as much as I can will help.

 

[Big Jim M]

Swap in a new pcv and check the hose for vac leks first before doing anything else. Don't mess with the timing until you KNOW for sure therein lies the problem.
Big JIm

 

[B2Mann]

Swapped in a new PCV and no change in idle. Hose to PCV is fine as well. How would I go about adjusting the timing?