1994 ford f150 4.9. Loose injector pulse at 1500 rpm

1994 ford f150 4.9. Skips a little at cold crank. Smooths out when warm, then when you give it gas it loose injector pulse at at 1500 rpms. It sounds like a drag race car with a splutter box. Cannot pull itself. I have anothertruck identical to it that runs fine. No problems what so ever. I have swaped map sensor, icm, distribator,icm, coil, pcm, idle control valve, egr, map sensor , plugs wire, injectors. I have swapped each part from one truck to the other. The good truvk continues to run good and the bad running truck continues to run bad. Helppppp

How do you know it’s losing injector pulse at that rpm?? Maybe it’s a bad spark plug In one of the cylinders. Have you performed a power balance test at that rpm and the cyl I that doesn’t make a difference is the suspect cylinder

Cheryl I checked at the number 1 and number 2 injector with a test light. They are on different banks of the injector pulse. Both start with a good pulse but as you pass 1500 rpms the pulse begins to fade, until its gone. I have checked the entire wiring harnes for breaks and have bot found anything yet. The injector pines on the computer are 58 and 59 so I checked the harness at these pins and get the same results pulse fading at 1500 rpms. I have several of these trucks so many parts at hand. I pulled the computer from a good running truck and put that computer in and got the dame results. So I put the computer out of the problem truck into the good running truck and it ran good with this computer. So It does not seem to be a computer problem

Do you have a way to verify that the PCM recognises the truck is in gear?
It seems logical for the PCM to not let you spin a rod out of the block free revving in park.

I think this will answer your reply. This model does not have a rev limiter. It also does this in park or drive. You can give it throttle very easy or all at once but at around 1500 the injector pulse starts to fade. I am open to any suggestions. Thank you for your help

Have you checked to see if your losing battery power to the power side of the injectors? Should be the same wire color for voltage to all injectors.

Yes I have checked power to injectors they are all on the same hot wire, 13v steady as a rock while the pulse is fading. Also checked the power to the coil same there good 13v . I have checked everything i can possibly think of but will listen to any ideas . I think this will be something very not normal so any help is appreciated.

Wonder if it’s something weird like to much ac voltage coming from the alternator.

I need to add also that it does not have enought power to pull itself. Fuel pressure is 55 lbs at idle and under power. Will start right up takes about two turns. Will set at idle for hours and runs perfect. I have six of these trucks and it is the best idling truck of the whole lot. It has the correct voltage to the icm, map sensor, tps. Checked the tps the voltage goes up when the throttle is open to correct spec and goes back down when throttle is closed. There are two power grounds one at the fender by the battery the other drivers side on the core support by the radiator. One computer ground at the drivers side by the hood hinge attached to the inside of the fender. All cleaned tested and in good shape. I have about 100 hours of work into this thing, so if you can think of anything i may have missed or just something to check please respond and let me no.

Cheryl could you explain ac voltage from the alternator.

Try voltage dropping the powers and grounds at the pcm at 1500 rpm backprobing the connector.

Sorry forgot to say this truck was like this when i got it. Someone was working on it try to fix it . It had new distribuator, wires, plugs .

Seen it in one of scannerdanner videos causing all types of weird issues on a Chrysler. It’s on the regular unpaid videos. He explains it much better than I could here

Cherl are you saying to check to see if voltage is dropping at the computer and check the grounds by back probing.

Yes voltage drop powers and grounds scannerdanner has vids on that if your unsure

Cheyrl checked the voltage by back probing the power and grounds at the computer. 13.5 to 14 v. Checking these when the engine is at idle and checking at 1500 rps nothing changed.

Voltage coming out ofthe alternator is in range 12.5 tp 15 v.

Wonder if it’s a timing issue? An actual mechanical timing issue causing the pcm to lose injector control

Wonder if it’s a timing issue? An actual mechanical timing issue causing the pcm to lose injector control

Hi cheryl, these engines use gears on the cam shaft and a gear on the crank shaft so no chain involved. I have worked on the 300 six for many years and have seen them running with two teeth missing on the cam gear and stay in time. But i have been thinging along the same line today but i believe for it to loose enought teeth to get out of time the timing will be so far off it would not run. I will add when i put new fuel injectors in i went ahead and pulled valve cover to check cam shaft. All the rocker arms had plenty of movement so the cam shaft is very good. That is as far as i have went into the engine. I think when this problem is found it probslly will not have anything to do with the injector pulse, something else bad or grounded feeding back throught rhe electrical system. But what, whished i new. Please keep thinking and posting it may give me ideas i have not covered yet. Took the fanbelt off today also to see if the alternator may be a problem but made no difference. So another dead end.