91 5.0 V8 Cold Start, Stumbles and Dies
 

I'm hoping I can get some help diagnosing my son's 91 F-150 Cold Starting problem. V-8 engine with 78 K miles.

Above 60 degrees, truck starts and runs great. No idle problems, accel problems, good mileage.

As the temp drops, the starting issue shows it's ugly head. Truck cranks and will fire, but stumbles and dies. Requires about 6 - 8 attempts, before it warms enough to keep it running by tapping the accelerator pedal. If you hold the throttle open to try to keep it running, it pops and bangs and dies, almost like it's crossfiring. Once warmed up, it idles and runs great.

I've searched this and other Ford fourms and here's what we've done so far.

1. Ran OBD codes... showed temp sensor out of range, replaced with new temp sensor (not dash indicator sensor, but the one that feeds the PCM). No other codes are showing up now.

2.. tested and replaced the idle air control valve. (actually on the 2nd one, as the first one from Napa tested bad). Checked connector, pins clean.

3. checked the voltage and steady movement of the throttle position sensor,
checked good and was smooth through the full range. Checked connector, pins clean.

4. checked the EGR valve for leaks and sticky movement, moves smooth.
Installed a temporary solid gasked under the EGR valve to insure no air leaks, no change. Reinstalled new EGR gasket.

5. Inspected vac lines and hoses. Used propane bottle method and found one small leak at the EGR vac connection, corrected but did not help cold start. Removed and plugged brake booster hose, in case of an internal leak, no change.

6. replaced ignition module, on side of distributor. I read some modules develop solder trace corrosion over time and one symptom was hard or cold starting problems. No change.

7. checked fuel pressure in rails and checked pressure regulator. Both checked good and were at the upper range of the scale 35-45 psi. Pressure went up approx 10 psi when the vac line was removed from the regulator.

8. Erased "keep alive" memory in PCM by removing battery cables and shorting them together, did not help.

9. Removed fuel injectors to checked for clogs or damage, found screens clean and injector tips in pretty clean condition. Ran injector cleaner in tank for three tanks. Don't believe it's a fuel problem.

10. Replaced PCV valve, mainly as a precaution.

11. Removed PCM and inspected and cleaned connections. Found white power (not corrosion) on some of the pins that appeared to be some dried up silicone grease.

12. Replaced sparked plugs (checked and gapped new plugs). Checked wires for signs of arcing. Cleaned and routed wires so #5 and #6 cyls can't jump spark. Replaced Dist. Cap.

13. Ran OBD for both Engine Off and Engine Running Codes, Shows Good, (no codes).

The starting problem has remained constant only when the temp. drops. The colder it has gotten, the longer it takes to get the truck warmed up and keep it running. Once warm, it runs great.

The only thing we found that made any difference... I suggested my son turn his key on (not starting it) and leave it for 5-10 minutes. Thinking the problem to be electrical, this may allow the faulty component to warm up.

This worked for him when the temp was around 50 degrees or so. The truck started right up and idled fine. But, as the temp has dropped into the 30s and 40s, This has not helped. He left it on about 20 minutes with the temp in the upper 30s and it stumbled and died multiple times.

I'm suggesting for my son to replace the PCM, but I wanted to throw out the problem on this fourm and see if anyone else may have any suggestions.

I appreciate the help from everyone (past threads). It's a great resource to have all this experience bear on a problem.

Many Thanks,
Frosty

Replace the MAP sensor.

afwflj, I would look towards the map sensor. Imo this sensor is the heart of the speed density system these trucks use. It measures manifold absolute pressure (the density in speed density) IME, It will not always throw a CEL unless its completely out of range. It would also explain why the truck runs/ starts better in warmer conditions. it sounds like its having a hard time sensing the correct density of the colder air. It is located on the passenger side of the firewall next to the hvac box.
http://i1229.photobucket.com/albums/...2611182604.jpg
Hope this helps.

Which V8? Fill in your profile so the truck's details show with every post. You can omit "F150" and "19.." since we assume that. But put the 2-digit year, engine size, transmission model, trim level, cab/bed configuration & wheelbase, transfer case model (if 4WD), gear ratio, tire size, miles (or KMs), mods, maintenance, damage, repairs, etc. If you run out of room, put it in a signature. If you don't know all the truck's details, read this caption & follow the links in it:

https://www.supermotors.net/getfile/...oorsticker.jpg

If there are any terms you don't understand, read this: Automotive Terms & Abbreviations

Uploading pics of your truck would also help. This forum supports pics, but it's easier to embed them in your posts if you follow the instructions at this link:
http://www.supermotors.net/forums/th...-and-or-videosWhich one? ECT or ACT? Did you test any of them?Changing parts isn't a good idea for several reasons; the biggest being: it's expen$ive and rarely works (as you're seeing). The only exception is the MAP, which is difficult to test, but common & cheap in junkyards.

The Haynes manual has an excellent ignition test procedure, but since you're past that, I hope you kept the old module, which was apparently still good.That's not a repair or diagnostic procedure. It's a driveability procedure AFTER a repair has been completed & confirmed, to erase all the bad data from the adaptions memory.Did it stink? Did you remove the case covers & examine both sides of the PC board?
http://www.fordf150.net/forums/viewt...p?f=21&t=97221Did you measure resistance through each wire? Read this caption:

https://www.supermotors.net/getfile/...ute8793_5l.jpg

bikertrash
If that's your truck in the (giant) pic above, you should repair that vacuum hose this way:

https://www.supermotors.net/getfile/...recircline.jpg

That electrical tape is unravelling when it gets hot, and allowing dirt directly into the intake.

Thanks for the help....

We opened the PCM and found it smelled burned. Didn't see any burned chips or capacitors.

Ended up replacing the PCM and so far, the truck is starting great, even on the cold mornings.

Will report back in a few more colder days.

Many Thanks,
Frosty

Yes Steve that's my truck in the pic. I have already repaired it by replacing it with one off my red truck. Thanks. Good looking out. It's appreciated.:thumbsup: