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HELP!!! 91 F150 has high idle

4K views 39 replies 4 participants last post by  mangumandrew 
#1 ·
it idles between 1000-1600 which is definitely high.
i have replaced
IAC
MAP
CTS
egr valve solenoid
egr purge valve
egr valve gasket
throttle cable
throttle body gasket
it came up with three codes from the check engine light (OBD1)
84) exhaust gas recirculation vacuum regulator( evr circuit failure)

51) engine coolant temp sensor cts indicate -40 degrees f/ circuit open.

54) air charge temp sensor (ait) indicates -40 degrees f/ circuit open

Do OBD1 systems store codes? or do they disappear when you disconnect battery?

Also motor shakes when runs. to fix this i replaced motor and tranny mounts, tailhousing (was cracked in three pieces) (also on my truck it mounts the tranny mount) The next thing i was gonna try is replacing harmonic balancer.

Heads up this is a 1991 ford f150 with a 1994 ford f150 motor swap.
the original motor was a 302 efi. Is there a difference in computers maybe? something that needs to get plugged in the prior owner forgot? is there anything you guys recommend me to do? i would like to keep my EFI system and hear this 5.0 purr like a kitten. the spark plugs when pulled out by me and my (more mechanically inclined friend that knows more about this stuff than me) mentioned its burning good.

i have the dual catalytic system in it, could clogged catalytic converters cause anything describes above?
 
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#2 ·
Check your grounds first and foremost. It could be your issue with 2 different sensors with the same failure.

Also, have you verified the firing order is correct? It should be the same with both the old and new motor but just in case it was swapped.
 
#4 ·
The firing order is not the same apparently it runs on a different firing order. If I try the other firing order it won't even start. Let me see if I can find the one that works

I'll have to locate the grounds for these circuits to make sure they're good.
You need to sort out these codes first before getting too involved , especially these which are sending wrong info to the ECU and throwing everything out

51) engine coolant temp sensor cts indicate -40 degrees f/ circuit open.

54) air charge temp sensor (ait) indicates -40 degrees f/ circuit open

what do you propose? ive already replaced the things in the circuit? i will be tracing the grounds. is there anything else i can do to figure out why it would be throwing the ecu out?
 
#3 ·
You need to sort out these codes first before getting too involved , especially these which are sending wrong info to the ECU and throwing everything out

51) engine coolant temp sensor cts indicate -40 degrees f/ circuit open.

54) air charge temp sensor (ait) indicates -40 degrees f/ circuit open
 
#6 ·
Yeah the newer motor should be 1-3-7-2-6-5-4-8, but if the ecu is the stock 91 ecu then that’s part of the issue. I would verify which ecu you have in the truck first. There should be a sticker on it somewhere.
 
#7 ·
it idles between 1000-1600 which is definitely high.
i have replaced
IAC
MAP
CTS
egr valve solenoid
egr purge valve
egr valve gasket
throttle cable
throttle body gasket
it came up with three codes from the check engine light (OBD1)
84) exhaust gas recirculation vacuum regulator( evr circuit failure)

51) engine coolant temp sensor cts indicate -40 degrees f/ circuit open.

54) air charge temp sensor (ait) indicates -40 degrees f/ circuit open

Do OBD1 systems store codes? or do they disappear when you disconnect battery?

Also motor shakes when runs. to fix this i replaced motor and tranny mounts, tailhousing (was cracked in three pieces) (also on my truck it mounts the tranny mount) The next thing i was gonna try is replacing harmonic balancer.

Heads up this is a 1991 ford f150 with a 1994 ford f150 motor swap.
the original motor was a 302 efi. Is there a difference in computers maybe? something that needs to get plugged in the prior owner forgot? is there anything you guys recommend me to do? i would like to keep my EFI system and hear this 5.0 purr like a kitten. the spark plugs when pulled out by me and my (more mechanically inclined friend that knows more about this stuff than me) mentioned its burning good.

i have the dual catalytic system in it, could clogged catalytic converters cause anything describes above?
I would look for a bent throttle rod or something along those lines.
 
#8 ·
I've got the idle down to 1300 rpms while idling, the truck runs good while driving but the idle is still way too high. I am gonna start diagnosing into the codes that are being thrown at me.

Do you guys have any references for wire circuits somewhere? I would like to follow the wires and make sure everything is connected as need be.

Once I get this idle down she will run like a champ!
 
#16 ·
There isn’t a consensus but either 94 or 95 was the first year of the 1-3-7-2-6-5-4-8 firing order. If the engine is out of a 94 with a stock cam then the issue is with the firing order. If that is not the case, then it could be a ground, vacuum leak or even the ecu.
 
#17 ·
Getting closer I added the ait sensor and it rests at 1000 rpms but when I bring the tachometer to 2500 and drop it it goes to rest at 1500.. what are the bad symptoms of a bad egr valve? Every other thing on that egr circuit is new but that. Maybe that could affect it?
 
#27 ·
That is the map sensor. It looks like someone took one of the sides off. I’d replace it and see how it runs. A faulty map sensor can cause issues from a no start to running rich with a high idle. Make sure the vacuum line from the intake to the map sensor is free of obstructions and is by itself. It’s imperative that it gets a clean vacuum signal.
 
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