Backfire thru the intake

1995 FORD F-150
130,000 MILES • 5.0L • V8 • 4WD • AUTOMATIC
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My engine backfires under a load, even more so at about half throttle. It somewhat goes away at full throttle, but sometimes it won't even rev up to get there. It's been suggested by a mechanic that it's is a valve guide problem. But if I unplug the MAF the backfire goes away, however the engine definitely is sluggish/less power.

I have replaced the, fuel pump & filter, spark plugs & wires, distributor (2 times) & cap & rotor, MAF (2 times), oxygen sensor, fuel injectors and the computer.

It is a OBDI so it's hard to get much information from the computer.
Aug 9, 2020 at 8:25 AM
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JACOBANDNICKOLAS
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Hi,

There are a few things that can cause a back fire. Take a look through this link and see if it helps:

https://www.2carpros.com/articles/engine-backfires-while-running

Also, I don't know if you checked, but here is a link that explains how to get OBD1 codes from this vehicle. If you have already done it, let me know what you found. If you haven't, do this and let me know what codes are found.

https://www.2carpros.com/articles/ford-lincoln-mercury-obd1-1995-and-earlier-diagnostic-trouble-code-definition-and-retrieval

____________________________________-

Let me know what you find.

Joe
Aug 9, 2020 at 8:16 PM
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I'll run the codes again but the mechanic I had look at it is a Ford guy, he even had a friend bring him a dealer shop computer reader to plug the OB1 into, and couldn't figure it out. He's the one that thinks it's the valve guides but if that is the case then why does it stop when the MAF is unplugged?
Aug 10, 2020 at 7:24 AM
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Oh yeah, we also changed the temperature sensor and the throttle position sensor. I'm really tired of guessing and buying parts. I wouldn't mind pulling the heads and having a valve job done but to me it's just another guess?
Aug 10, 2020 at 7:26 AM
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JACOBANDNICKOLAS
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The MAF sensor measures the total volume and mass of air flowing into the engine. It is one of the major factors used by the Power-train Control Module (PCM) in determining the fuel injection base pulse width and ignition timing advance. When you disconnect it, the system goes into a predetermined setting which is likely causing less fuel to enter the engine.

This may sound crazy, but have you checked the fuel pressure to make sure it isn't excessive? Pull the vacuum hose of the regulator and see if it is dry or if there is evidence of gas in it. Also, let me know if you have checked pressure.

Let me know.
Joe
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Aug 10, 2020 at 10:10 PM
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I have checked and replaced the pressure regulator. I didn’t check the pressure but the mechanic did and said it was fine.
Aug 11, 2020 at 4:42 PM
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JACOBANDNICKOLAS
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Hi,

At this point, I feel like your friend is on the right track. It could be a flat spot on the cam, a valve guide issue, burnt valve and so on. Has engine compression been checked?

Joe
Aug 12, 2020 at 9:15 PM