My truck has miss. tried everything (just about) unplugged map sensor, truck runs fine. bought & installed new map sensor. Made no difference, but with new map sensor unplugged, truck runs fine, plug in, truck misses. Truck is 4.9 300 6, 1990 F-150 auto. 4x2. 79,000 miles. Any ideals? Thanks!

 

interesting, but i'll bet that its something about limp mode when you unplug the sensor. have you pulled codes from it?

 

Chech enginge light has not come on except when map unplugged.
Also, all Ive done is on another thread. Here it is

Ok, this is my first post. Tried to look at other post, have found no solution to problem. 1990 f-150 300 6 auto. 4x2 short bed. Just bought truck. Has 79,250 miles (actual). Has a dead miss at all speeds & at idle. Not continuous, but constant. Compression at 148 to 150 on all cylinders. Replaced so far, injectors, fuel filter, plugs, wires, rotor, cap, stator assembly, and coil. Think that’s everything. Tried the egr plug fix, no good. Thanks, any help appreciated.

 

Can you pull codes if check engine light not on?

 

checked connections, seemed good. Check engine light has never come on. But, I unhooked map sensor. (Not vacuum) just electric plug. As soon as I did, it ran great. Ran down road great, so I replaced map sensor. Didn't help it one bit, but I unhooked new map sensor, truck ran great. Not a single miss. Drove it about 15 miles, ran great whole time (never ran like it before) pulled in driveway, hooked up sensor, started missing immediately. Check engine light on as long as unhooked, re-hooked, check engine light goes off. Only problem when map sensor unhooked, truck shifts really, really hard. I don’t have a clue. <O</O

 

i would go ahead and check the codes, it is possible to set codes and not turn the light on.

it definitly sounds like its map related though. the tranny uses the map to know engine load so it knows when to shift. when the map is disconnected and is in limp mode I believe that it uses rpm for shift points.

have you traced the vacuum line back to the engine? it should go back the manifold for a direct vacuum reading.

another thing that wouldn't be a bad idea (but more labor intensive) to do is load check the wiring for voltage drop.

 

Can't get code reader to even register on truck, won't do anything. Vacuum is fine (no leaks) Also, was told it was probably the throttle position sensor. So, I bought it, put it on, no difference, but if you unhook the tps, the truck runs great. So whether it’s the map unplugged, or the tps unplugged, truck runs great.( no misses) I’m no mechanic. don't claim to be.Now I’m being told it most likely the ecm. <O</O

 

Once again, get the computer codes. The following sites tell you how to do it without a code reader:

Ford Fuel Injection
Ford Ranger/Bronco II EEC-IV Testing

 

Hummmmmm.............

Well, heres something you could check.
A9x ECM's (and same years ECM's) Failures Due to Age
This worked for me.
It cured a stumble I chased for a couple of years.

Good luck !!!

Bob

 

Been out of pocket for a while. Could not get computer to do anything no matter what we tried. (except cel would come on if something unplugged). Took eec out of truck. It had the wrong one in it. Took over week just to track down right #s. Got one ordered, put it in, NO more miss. Truck is running great!! Thanks for all the input. <O</O

 

Wow !!!!!!

I guess the PO thought an eec is an eec is an eec ............

Glad you found the trouble though.

It's tough throwing cash at a problem someone else created.

Bob

 

Limp mode was programed on those old ECM???

 

Limp mode was programed on those old ECM???

 

Yes. As far as I know all Ford EFI systems have a limp mode.

 

It is possible re-curve a MAP sensor? is there any aftermarket one?