just wont stay running HELP!

[justanotherbroncodude]

Here is the problem, my sons 88 bronco 4.9l efi bucks and dies at around 25 mphs every time does not matter what gear we are in. Does not seem to be fuel related. We have replaced fuel sending unit, fuel filter, ICM, coil, coil pigtail, plugs, plug wires, battery, battery cables, we have checked all suspicious wires, and have taken it to the garage where they could not diagnose the problem.......any help would be greatly appreciated

 

[50joe]

Thats wierd. It only does this at 25 MPH, or is it a RPM related issue? How does it run at idle/ no load rev? If you haven't done so already, you may want to check ALL your grounds. (ESP. the smaller ones on fender or rad support). I have seen a few late 80's early 90's Fords act up out of nowhere, turned out the grounds weren't making a good connection, throwing off the ECM.

 

[justanotherbroncodude]

Thank you for the reply......Battery ground wire was cleaned positive side replaced,checked and rechecked all other grounds, it idles and revs up perfectly and I originally believed it to be a problem that occurred under load which seems to not be the case, I don't know if there is a speed sensor or governor that might be a problem could not find any info on that

 

[50joe]

It does have a speed sensor, (in the rear diff), but I have never heard of one cutting the motor off. Is it a manual or automatic trans? (guessing manual?)

 

[justanotherbroncodude]

It is a manual

 

[50joe]

Thats wierd it cuts off at 25. I wonder if there is something out of wack with the speedo itself? I know it's cable driven, but maybe the spring on the back is making contact on something at 25ish? All that speed sensor is for is the ABS. Does it start right back up once you stop?

 

[justanotherbroncodude]

It does start right back up with no problem at all

 

[50joe]

Thats just crazy. I would start by popin the cluster out and takin a peek. Maybe try runnin down the road with the cable off of the speedo. I have never heard of anything like that, maybe someone else on here has. Either way, would love to know what you find.

 

[justanotherbroncodude]

thanks for the help will try that and will definitely give updates, seems to be a real puzzle

 

[bobstrat]

do you have to have the clutch engaged to start it?

 

[Chrish_jr]

I had a celebrity years ago that would do the same thing, ECM went out on it causing it. Its not the same type of vehicle, but I would pull codes and see if it may be the ecm at fault. The ironic part about that was, 4 service shops told me the engine needed to be replaced. I couldnt afford it, so pulled out the code book and since no two codes came up the same after several resets of the ecm I decided to go to a local savage yard, pulled an ECM, car ran perfect for years later and 3 new owners lol.

 

[justanotherbroncodude]

I do have the clutch engaged when starting.....The ECM will be the next on the list of things to swap out