I have an '87 F150 300 six with an idle problem. When the engine is cold, the idle goes low enough that the engine stalls out. It cycles between 1200 and 600 until finally stalling. Improves somewhat when warms up but is very unstable. Especially annoying while trying to manuever in a parking lot.
I have cleaned the throttle body and idle bypass with solvent and brush. No vacuum leaks found. Installed a new TPS and sparkplugs.
The manifold pressure sensor is getting 5V. Feed back is 3V and does not change with vacuum. Does this vintage sensor work on voltage or frequency?
The inlet air temp sensor resistance changes with temperature so think it is functioning properly.
Any ideas? How do dirty injectors act?

 

I think you have found a bad MAP sensor. Try unplugging the electrical connector while idling to see what happens, if it runs better, you got a bad MAP.You may not be finished though, check for continuity across the IAB solenoid,should almost show a direct connection like 10 ohms or less.Good luck !
Phillip

 

I disconnected the MAP sensor and idle smoothed out. I am confused on the diagnostics on this sensor. Did Ford make two types of MAP sensor, one using a voltage feedback and the other a frequency feedback? How do you know which one you have?

I will purchase a new sensor Monday and hopefully the problem will be take care of.

Thanks, Kevin

 

I have now replaced the MAP sensor with no improvement. If I disconnect the electrical connector, motor idles fine at 1200. Plug it back in and idle is fluctuating and engine will stall out..

Tried a new vacuum line between manifold and sensor with no improvement (was told some lines delaminated and could suck shut).

Also tried another IAB with no improvement (both check out fine electircally).

Any other suggestions?

 

How old is the coil ?

Check the primary and secondary coil..

I can tell you how to do this if you need help...

I had that porblem today, because I washed my engine and I soaked my coil big time and until I got the engine really hot (160), it kept missing..

Also check lose spark plugs cables, spark plugs and wires to the coil...

Keep me posted..

 

Spark plugs are new. Wires are one year old (autolite). No evidence of corrosion at the distributor cap and coil.

Coil is original. What do I look for in evaluating the primary and secondary coils?

Thanks, Kevin

 

What is you e-mail account ??..
I will scan the three pages from the book and I will send them to you

 

My email is klhwarf@nb.net.

 

Kevin: a couple of quick questions. Have you checked your timing? I have an 89 bronco with a 5.0. I advanced the timing to 15 before to give it a little more hp. After I did this it acts about like you discribe when I first start it cold. Also do you know the background of your motor? I installed an r/v cam a couple of years ago and I think the computer and the cam are arguing over who is boss. I'm not sure if it started with the cam or the timing. Is there a chance that you have an aftermarket cam? Mine seems to run fine after initial warm up. Maybe this will help.

 

Kevin,are you sure the MAP sensor you purchased was a new,uninstalled one out of the box? Did you look for the screw head marks that would indicate a previously installed MAP sensor that got returned as a defect and possibly placed back on the shelf as a new one? I have witnessed such things before,first hand. After the customer walked out,the counterman makes a comment that the customer is having other problems and decides the part is still good and places it back on the shelf! The only other thing I can go on is possibly the wire harness for the MAP is bad such as a short or bad continuity.Check them from the connector back to the computer.Also,check out the ECTS(engine coolant temp sensor)for function,resistance and continuity.You may have to check into fuel pressure and a bad module/pick-up also.
Phillip
Phillip

 

I shouldn't have been so quick in condemning the new MAP sensor. The night I put it on, I couldn't see any difference. When I started the truck, idle was still unstable and truck stalled.

However, it has since been behaving much better. Sometimes stalls once as soon as engine is started but restart and idles okay.

It is funny you should mention about looking at the MAP sensor when you purchase it. I was getting ready to leave the store and I looked at the number on the new MAP sensor and it didn't match the Ford cross reference they had given me. Turns out part had been switched with another p/n sensor. Saved me a trip back to the store.

Thanks for your input.

 

I had the identical problem. The idle would fluctuate between @550 to 1000. On advice from the dealer, I replaced the MAP sensor and it went away. Good luck.

 

Have had very similar problems with my 87-300. Tried all the initial checks. Throttle body cleaning, air bypass etc. Had no codes so I was stuck. Turns out the EEC is bad. Guess what. Ford doesn't make them anymore. However, aftermarket EEC works just fine and is about half the price of the OEM one. The other thing that is important is to disconnect the battery and let the computer caps discharge. This allows the processor to learn the new input from what ever sensor you have replaced.

 

The MAP, manafold absoulte presure, sensor measures the vacum and outputs a variable frequency square wave signal to the computer. I measure the output signal with an osciliscope.
The connector, three pins are ground, 5volts fron the computer, and the signal.

The square wave signal frequency should vary with changing vacum, you can even get it to change by sucking on the vacum line to the sensor.

ALSO... try reseting the computer as somestimes it gets confused trying to stablize the idle..
Unplug the computer module connector, with the key off, and leave it disconnected for about ten minutes to drain the "memory". Then reconect the computer and let it "re-learn".

 

Can you also disconnect the battery with the same results? I have tried it this way and it seems to work after changing some components or even a tune up. It does seem to take a tank of gas or so after that to regain full performance. My fords are eecIV 88 to 91(2-5.0s and 1-2.3 turbo)