Hey guys! I've been lurking/searching for a bit and haven't been able to find a solution that works for my specific problem. A little back story on the truck--it's a 1990 F150 5sp. 4x2 with the 4.9L. It shows around 80K on the odometer but the previous owner wasn't sure if it had already flipped over once, he got it from his grandpa. Anyways, I've owned the truck for the last 10 years and only use it for work on the weekends.

In that time, it's had a new distributor, coil, plugs and wires, and routine maintenance. A few months ago the engine died while driving home through the neighborhood. It turned out the be the PIP sensor in the distributor. I replaced it with a new Motorcraft sensor and it ran again. Recently, it has began to be harder to start and missing badly under light to medium throttle but clears somewhat once wideopen.

I checked the timing and it's set at 10 degrees. I ran a KOER test and it showed fault codes for Map sensor (72, Throttle position sensor (73) and knock sensor (25). I replaced the Map & throttle sensors, and oxygen sensor--and surprise it still ran terribly. I checked fuel pressure at the rail and it showed 40 psi, the manual says it should be at 45–could that be my problem?

I've checked and as best I can tell there isn't a vacuum leak. so... what am I missing? I'm done with guessing and replacing parts. I'm not a great mechanic but can follow directions. Is anyone yelling at there screens saying "check the _______ stupid?"

Any help is greatly appreciated!

 

Still could be the fuel pump. About 3 months ago my pickup was doin the same thing. Mine was running ruff when starting also. I automatically thought electrical..maybe timing. Had changed pick up coil in distributor a year before. Left distributor bolt loose to adjust timing if needed. Ran good..4got about loose bolt. That wasnt the problem so I changed the ignition coil. Over the past 5 yr Ive changed a lot of the electrical components..coil was about all that was left. Changed it..didnt help. Got to lookin around online. Some dude had the same symptoms as mine..he was changing electrical parts..not having any luck. Another guy told him..with confidence.."its the fuel pump". Told him that he could replace every electrical part on the truck..it wont help. He had gone through the same thing..same symptoms..didnt get anywhere. Finally changed the fuel pump..truck ran like a champ. That man was so confident about what he was saying..I ordered a fuel pump rite then. Figured it couldnt hurt anything..probably still has the factory pump. If it dont work..at least it has a new fuel pump. Got the new fuel pump..installed it..runs like it did b4 all the skipping started. I didnt pressure check the fuel rail. If it runs good at times..like mine did..of course the fuel pressure would be what its supposed to be at those good running times. There is probably some physics..thats over my head..the way the engine running..or not can cause the fuel pump to act different. All I know is mine was gettin very aggrivating to drive..skipping..then run ok. Mostly at starting engine and taking off from a stop or slow roll. Even thought I had water in my tank. Added drying stuff to gas. No luck. That fuel pump did the trick. I cant say to you with as much confidence as dude had that a new fuel pump will solve your problem..but it sure solved mine. And if your truck belonged to me..thats the 1st thing I would try.

 

Thanks for the reply! I decided over the weekend to go for it and swap out the fuel pump. I was out of ideas of anything else that might be wrong. I had some help and got it done in about 3 hours, of course the tank was full. Long story short, I hit the starter and it fired up fast like it was new. I took it for a quick spin and the skipping was gone–woo hoo! Amazing that the truck would still run with the bad pump but just couldn't get up to pressure. Thanks for reinforcing what I was thinking! I might not have went for it otherwise.

 

Shoot..that was about $200 worth of info. I aint goin to charge you but $100. Man..I might know that same info 20yr from now if needed..but I didnt invent it. I found it rite here on Al Gores internet just like you did. Most things..no matter what it is..I can enter it in a google search..my question mite be 2 sentences long..and more times than not something very similar to what Im lookin for will come up. Rephrase my question and hit the nail on the head. My being able to remember what someone else told me..dont impress me. Im impressed with the 1st person that came up with the info..then shared it. Being able to filter through all the bs online..Im not sure if thats luck or skill. After dealing with that skip..havin it fixed..It does feel like drivin a brand new pickup. Glad you got it goin.

 

Your 4.9 should have 45-60 psi of fuel pressure. I bet you are well into that range now. I hope you changed the fuel filter too.

 

I'm guessing so also. I thought about borrowing the pressure gauge again from O'Reily but it requires a $175 deposit . I changed the fuel filter a few years ago and have probably put less than 10k miles on it. Is it common practice to change the fuel filter when installing a new pump?

Next project up on the truck is to replace the blown out air shocks on the rear. The back end almost becomes airborne when I go over a speed bump...

Thanks for the input guys!

 

$175???? I would just buy a gauge. They are around $35-40 and should be in anyone's tool box who works on cars. Also, it is always a good idea to change pumps and fuel filters together. Technically, the pump warranty would be voided if you don't change them together.