I have a 1990 f 150 4.9l with 290,000 miles on it. I had problems passing emissions so i changed the o2 sensor, no help, then changed the egr valve, passed emissions. But know i have a skip that has started and is getting worse. Ran better before i change the egr valve. runs good at idle. Runs good untill you reach about 25 mph then starts to skip a little

 

Usually I recommend looking at the EGR valve, but since you just replaced it...

Couple of things:

Clean the throttle body and intake manifold with a throttle body cleaner. Carbon buildup on the throttle can cause a slight skipping.

Pull and inspect the distributor cap and rotor button. Any buildup or corrosion present? If so, clean or replace if necessary. Let me recommend replacing as my experience shows cleaning only fixes the problem temporarily.

Lastly, pull the codes and report back. Do this first before diving into sensors, valves and controls. The above two can be done for very little money ($3 for a can of cleaner and you can pull and inspect the distributor cap for free), every thing else starts to add up.

 

Ok. so let's check the basics. When was the last time you did a complete tune-up? If it's been awhile then it may be time for a new cap & rotor (Brass tipped not aluminum), Autolite spark plugs, plug wires and filters (fuel & Air)? Also are all of the plug wires routed correctly and clear of any potential grounding? Sometimes they'll have a tendency to cause a miss if they happen to be too close or make contact. Check it all out and let us know how you made out. Welcome to FTE and Good luck!

 

Thanks Guys, i will try to get to it this week

 

changed the fuel filter no help. check codes no codes found. looked a #1 spark plug looked good. vaccum leak going to the egr valve?

 

Vacuum leak is a certainly a possibility. Go over all hoses and check, not just the EGR. Also, pulling the #1 plug doesn't tell you anything other than the #1 plug is fine. You need to check the others as well. Since you don't have any codes registering, I'm still recommending you clean the intake and throttle body real well and pull the distributor cap and inspect the contacts and rotor button. Replace if need be. Also inspect all your plug wires for a good seat on the plug and no breaks/cracks.

 

Thanks for all your help. i am going to try to look at it again today. Randy

 

sorry it took so long been busy. sprayed and flushed intake with carb cleaner. replace plugs, plug wires, rotor, and cap. No Help Thanks Randy

 

Check the #7 and #8 plug wires and make sure they aren't touching. These two are known to crossfire and cause a skip. Reroute them and make sure they stay apart. May not solve the problem, but it's a start.

It also may be a bad EGR valve. I know you replaced it but it could have been faulty from the manufacturer. You may want to drive it for some distance and pull the codes again. Are you pulling KOEO and KOER codes? Both are necessary to get a good sense. Usually, a noticeable skip will throw a code.

Also, use throttle body cleaner to clean the TB and intake. Different molecular structure than carb cleaner as it's a different surface (carb being metal, TBI being alloy).

Pull some seafoam through the MAP vacuum and clean the intake and injectors real well.

 

thanks chewie i am pulling both KOEO and KOER codes. did use throttle body cleaner. i will look at he wires for 7 and 8. can i disable the egr valve some how?

 

You can block off the EGR valve which is essentially disabling it, but I've never done it before. Essentially it requires taking some light gage aluminum and cutting it to the size of the gasket. Then use some silicone or another gasket to seal it. Apparently you'll want to leave the EGR valve on there to hold the block in place. If you'll do a search on the forum you'll find some more info. Search "Block EGR".

I'm still not sold on the EGR because it will almost certainly throw a code if problematic. Do you notice the skip/hesitation when both hot and cold?

Since we don't have codes...let's look at fuel. Check your pressure at the rail. Should be 36 at idle and 40 WOT. The injectors may be clogged up and not getting enough fuel. Also, do a compression test on each cylinder (wet and dry). A leaking head gasket could cause a hesitation. Any oil in the coolant? Discoloration of oil? It could also be bad/stuck rings. This would usually cause blow-by and result in fuel in the oil. Does the oil smell clean or does it smell of gas? You could also have a bad/stuck valve. With the high miles, any of these is very possible. A compression test can help pinpoint.

Sorry to be throwing so much at you, but I'm just trying to rack my brain for anything that could cause a skip.

And you and I are going to feel silly. Your truck won't have the #7 and #8 wires since it's the 4.9L I6. I knew this before, but the #7 and #8 plugs are such problems on the V8s my fingers out-thought my brain.