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My brother in law has a 99 f-150 with the 4.2l v6. It had 2 vacuum leaks that I fixed. He had put new plugs and wires in it and it ran like crap. I pulled the plugs and check the gap. They were all at .040-.045 and the gap should be at .052-.056. I gapped all the plugs to .054 and reinstalled them. Reinstalled the plug wires and they felt like they were a little loose on the plugs, they all clicked in place but had some movement. The truck ran and idled great. Just got a call from him and he said that it sounded like it was back firing through the intake. It only does it when you put a load on the engine. Anyone have any ideas or suggestions for me? :dunno:
 

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backfiring thru the intake could be egr valve stuck wide open, and would cause running rough, they are bad as any to get clogged , low fuel pressure possibly , could also , how many miles
 

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backfiring thru the intake could be egr valve stuck wide open, and would cause running rough, they are bad as any to get clogged , low fuel pressure possibly , could also , how many miles
Not sure how many miles. Never thought of the EGR valve. :doh: He did say that he was getting terrible mileage and that could contribute to that. I will find out the mileage and pull the EGR valve. I really appreciate the reply and suggestions. :nod:
 

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backfire through intake usally indicate a lean condition also. I'd look for another vacuum leak. hope this helps.
 

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CEL on??? Any previous codes??? That truck has a misfire counter that can be checked by either Ford or if you know someone with a current Snap-on scanner(or something similar) you can access mode 6 data. May not hurt to clean the MAF too. They can cause what you're dealing with too. Good luck
 

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Ok, new development. I pulled the plug wires back off and checked them. They weren't fitting right so we got a different set and reinstalled them. The truck idles perfect but has a hesitation off idle when in park. When you put it in drive it falls flat on its face. Almost like it isn't getting enough fuel. The truck has 100k on it and to my knowledge the fuel filter has not been changed. I am going to put a new fuwl filter on it and see what happens. My question is, what is the fuel pressure suppose to be at for this engine? After I replace the filter, I want to check fuel pressure in case the pump is weak. Does anyone think that this is a possible cause for this problem? Appreciate any info.
 

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EGR is what id look into, disconnect the vaccum line to the valve and plug the line. Then raise the engine speed. If it runs fine after that look into carboned up passages.
 

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EGR is what id look into, disconnect the vaccum line to the valve and plug the line. Then raise the engine speed. If it runs fine after that look into carboned up passages.
I did disconnect the vacuum line and checked it. It still idled perfect but when you put a load on the engine it fell on its face. Has a terrible hesitation right off idle and extremely low power. Should have put this in the last post but it was late and I was tired. Thanks for any help.
 

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Discussion Starter #14
Got it fixed. Found the drivers side cat was clogged causing the problem. Replaced the cat and truck runs better now than it did when he bought it. Appreciate all the tips.
 
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