I have a 1993 Ford F150 4x4 5.0. I recently bought this truck for cheap and now starting to realize why they sold it. The issue I am having is that when I start the engine cold it has a very eradic idle, like surging for air and sometimes dies. When you try to give it gas it acts like it is missing. When it warms up problem goes away but sometimes backfires/runs rough driving then goes away.

Here's what I did, Just did a tune-up(plugs,wires,cap,rotor) the motor has 100k from ford crate. It has new map,iac,egr sensor..etc. all sensors have been replaced.

I do have cel codes and I will have to pull them again as I forgot them but it was 33,34 and 22 I think. I did test the sensors and they seem to test fine. But my question is when I test the vref signal on the map sensor its 5v and then I test the map sensor return wire its 5vs.. when pluged in its 3.4vs sending to computer. Is this telling me I have a short or a bad pcm?

Anyone have any ideas? Im starting to lose my mind on this one! About to pull it and throw in my 4.9L

 

It's possible you have a wiring or PCM problem, it may be a good idea to pull the PCM to see if in fact it's the right one for the powertrain combo you have, and if there are any obvious problems with it like exploded capacitors.

 

You should be getting three digit codes. I would also expect this truck to use MAF instead of speed density. Are you sure your truck has a MAP sensor versus a MAF?

 

Ok, hopefully tmr night I will be able the pull the computer out. Do these computers usually go bad or is it a rare occurrence? I've been doing a lot of reading and also have a lot of eec-iv books, from what im getting a the vref signal is 5v and the map wire is the sensor feedback to computer and signal return is essential ground. Do all the sensors on the efi share the same ground?

 

The title says its a 93 but the door sticker says it was manufactured in 92, so im wondering if I have a inbetween year change.. I have a craftsman obd1 and obd2 scanner that i have been using to pull codes. Its a map sensor and its speed density. I was able to pull codes from both KOEO test and KOER tests. Recently I cant pull the KOER test as it gets half way into the test and the engine dies. I think its when the computer bumps the idle up to 1600 rpm during the test. My scanner then ****s up because the engine died. I'm leaning towards the pcm but is there any sure way to test the computer or do i need to get one of those star rotunda devices? I just started to mess with efi and its a learning curve for sure!!

 

Stick with it, I know from experience that the fuel injection system can be a pain but when it is in proper working order, it is an awesome system. As to your question on is the computer a common problem, yes and no. In the Mustang world you hardly ever see computers go bad, but in the trucks I've seen plenty. I've had to replace them in my Bronco and F250, a good buddy of mine had to replace his in his F150, and I've even had to replace a couple for customers.

 

your subject line says 1994 then your opening statement has 1995. Very confusing. A 1993 model year will be OBD-I with Speed Density but the codes can be two or three digit.

 

All of the sensors use the same ground. The ground is bridged from sensor ground reference (SIG GND pin #46) to pins 40 & 60 inside the computer.

The 1993 year starts 3/4 of the way through the 1992 year. You go by the tenth place in the VIN for the year of the truck and would be a "P" for 1993.

The 5VDC (V REF pin #26) comes from inside the computer and goes to three places. It goes to the MAP, EVP and the TP sensors.

Sounds like you have a lean mix. Maybe the EGR valve is stuck open.

No there is no way to test the computer and the star rotunda device will not help you test it.
I think as I do not have one that it is just a scanner and can read the sensor output values and the codes. It can also hook to the ABS computer on some vans with an extra cable set.
I have a after market scanner that will do the same thing except it will not read the ABS codes.
Note I am not talking about RABS on the F-series trucks when I say ABS.
I am not sure about what you have but I think it is just a code reader and not a scanner.

 

Sorry everyone, was half asleep last night was a long day. It's a 1993 but will check Vin to verify.
88lx5.0h,ok didn't realize that. I've had a 620hp mustang with maf 1992. Never had pproblems.

Subford, I've probably have a code reader then it can do all the ford tests but can't read sensors. Thanks for the pin outs will have to verify wires and ground. Now when the engine is running I pulled the vac of the map sensor and it did nothing to the engine. Now from what I read the iac and map control the engine when "cold start". Would it be fair to say if I pulled the vac or the plug it would affect the running condition? I've also pulled the egr plug to when running nothing happpend.

 

The EGR does nothing while the engine is idling. When you say you pulled its plug are you saying you pulled the vacuum line of the EGR valve, the electrical plug off the EVR or the EVP?

The Map sensor is not used at idle.

 

I pulled the egr vacuum line off, electrical connector at evp. When should I test the egr? Above an idle and not wot? What sensors are used to control the idle. Also I forgot to mention that when it runs rough it's blows some black smoke. Not a lot but noticed it on my newly paint garage door ugh.

 

It always reads 3.4v when running?

 

Go to the link below and about 2/3 of the way down it will tell what sensors are used when.
Fuel Injection Technical Library » EEC Computers

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Sorry to bring up an old post but figured id update it for anyone who had the same problem. It ended up being the egr. I did an egr delete and the replaced the map sensor ran great. I think the egr was messing up the return signal which caused the map sensor to not work. Hope this helps someone as it took a couple of months to figure out. I even tired replaceing the egr sensor.