Bruno2 Posted October 27, 2022 Share Posted October 27, 2022 I am having issues with a 91 F 150 351W. It seems to act up when it's cold outside. Sort of like the fuel and air isnt mixing right. I know these trucks have an ambient air temp sensor, MAF and a MAP. I believe it is some sort of communication error with the PCM. Is there a procedure to ohm the above sensors out for diagnosis? Symptoms are hard starts. Truck turns over, but doesnt like to catch and run. When it does finally run it has a horrible idle, smells rich and wants to die at stops.Requires a lot of throttle to clear it out and make the truck go. Any suggestions would be great. Link to comment Share on other sites More sharing options...
85lebaront2 Posted October 27, 2022 Share Posted October 27, 2022 There are resistance values for the sensors, ECT, ACT are resistance, the MAP is a strange one, it generates a variable frequency based on the manifold absolute pressure (amount below atmospheric). FWIW, the ACT and ECT values are the same for a given temperature. Here are the values, if you have a good multimeter you can check the values on the air and coolant sensors. Link to comment Share on other sites More sharing options...
Bruno2 Posted November 29, 2022 Author Share Posted November 29, 2022 So I thought we had it whooped after changing the MAP sensor. Then I decided it was an ignition issue. I put the spark tester on the coil and it was hitting just fine. So it's not the p/u coil or tje ICM. I guess I need to pull codes now. Link to comment Share on other sites More sharing options...
Gary Lewis Posted November 29, 2022 Share Posted November 29, 2022 So I thought we had it whooped after changing the MAP sensor. Then I decided it was an ignition issue. I put the spark tester on the coil and it was hitting just fine. So it's not the p/u coil or tje ICM. I guess I need to pull codes now.Here's the video Brandon sent me: Link to comment Share on other sites More sharing options...
Gary Lewis Posted November 29, 2022 Share Posted November 29, 2022 Here's the video Brandon sent me: That sounds like late timing to me. Maybe the timing chain jumped a tooth? Link to comment Share on other sites More sharing options...
85lebaront2 Posted November 29, 2022 Share Posted November 29, 2022 That sounds like late timing to me. Maybe the timing chain jumped a tooth? I agree with you Gary, definitely sounds like it is quite a bit retarded. Have you tried a vacuum gauge to see what you have for manifold vacuum with it running? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted November 29, 2022 Share Posted November 29, 2022 So I thought we had it whooped after changing the MAP sensor. Then I decided it was an ignition issue. I put the spark tester on the coil and it was hitting just fine. So it's not the p/u coil or tje ICM. I guess I need to pull codes now. Without knowing a thing I replied to Gary's video asking if the truck had good fuel pressure and if the regulator vacuum line was dry. I didn't get the first impression that the cam timing had slipped. I'm not hearing it spitting back, but that may just be because it doesn't have a carb. I hope it's a simple fix! Link to comment Share on other sites More sharing options...
Bruno2 Posted December 1, 2022 Author Share Posted December 1, 2022 I agree with you Gary, definitely sounds like it is quite a bit retarded. Have you tried a vacuum gauge to see what you have for manifold vacuum with it running? It looks as though I am 9-10° before TDC WITH 15-16 Hgz of manifold vacuum. Link to comment Share on other sites More sharing options...
Bruno2 Posted December 1, 2022 Author Share Posted December 1, 2022 It looks as though I am 9-10° before TDC WITH 15-16 Hgz of manifold vacuum. Also, I believe the truck has two fuel pumps(one in each tank) and.it acts the same with either one selected. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted December 1, 2022 Share Posted December 1, 2022 Also, I believe the truck has two fuel pumps(one in each tank) and.it acts the same with either one selected. Don't you have a lift pump in each tank and a high pressure pump at the reservoir on the driver's side frame? Link to comment Share on other sites More sharing options...
Recommended Posts