Truck runs like crap when idling cold. Lots of surging and or missing it sounds like. Soon as the pedal snaps back after the cold idle switch turns off it runs just fine. Truck is pretty high mileage I suspect it could be injectors but not really sure. I’m likely gonna order some new fuel return rails, injectors and bypass style fuel system from R&D IDI next payday but was still curious if anyone could help determine the cause? Also I recently replaced the mech lift pump.

 

Is it only on first cold start? Air intrusion from sitting overnight will cause the surging/missing like you're describing. Do you have a manual cold idle "switch" you're shutting off and it suddenly idles fine?

 

I disconnected that cold idle thingy several years ago.
I don`t like an engine to suddenly rev up when cold.
When I figured out what was making it to that, I unplugged it.
Have never had a problem with it idling when cold. Some
mornings it does get down into the teens in the winter.

I agree you might have an air intrusion problem, and the fuel
drains back towards the tank, and has to get pumped back
to fill the fuel lines and Filter etc...

When you replaced the Lift Pump, did you replace the rubber curley Q hose?
I had an air intrusion problem, and it was that hose. They get old and crack
where they are clamped to the pump and steel line.


Charlie

 

Depending on the temp do the glow plugs cycle? If it's just the first minutes while hi idle, I'd think temp/gp's. Does the engine run rough when you increase throttle to match cold rpm idle?

 

I think it may be only on first cold start not sure. No manual high idle it’s automatic.

 

I didn’t replace that hose but I did replace a steel line. I think I stripped the nut and instead of going through the hassle I just put in rubber fuel line. I was looking at that hose pretty suspicious though I’ll double check and replace it if I didn’t already.

 

my glow plugs no longer work as the previous owner/(s) have really messed up my electrical wiring. Guy said they don’t work and gave me two sets the old ones and new ones. Will miss beats when adding rpm while cold but clears up. I also have this problem if when I floor it in first or second gear it starts to run bad and smoke blue like crazy but clears up in a second.

 

If your glow plugs don't operate and you're able to get it started cold, it is going to sound terrible and have missing cylinders until it warms up.

You using the block heater?

 

ya I keep it plugged in 24/7 and still idles bad. After it levels out after start it sometimes sounds like I have a really lopey cam. My truck won’t start if it’s not plugged in or at least will take a long time. Don’t like doing that to the starter. Also have an oil heater that I keep plugged in all the time too.

 

I forgot, I also replaced the rubber line to the Fuel Pump.

Forget the old used Glow plugs from the guy you bought the truck from, toss them.
Use only MotorCraft Beru plugs. The others may be cheaper but will fail in a short
time and known to swell up and stick in the head.
Your 6.9 will use the ZD-1 plugs unless it is the 87, they changed the GP Relay/Controller
and use the ZD-9 plugs. 87 last year for the 6.9.

Strange when the Block heater is plugged in it runs rough, should start like a warm engine
and be smooth.

You mentioned the GP wiring is messed up.
If your 6.9 is a 83 - 86 with the old style Controller and GP Relay over on the passenger fender,
you can bypass and use a manual spring loaded toggle switch. Connect a hot (Positive) with
key on to one side of the switch. Off other post of the toggle, run the wire over to the GP Relay
and connect to where the purple wire connects. Remove the purple wire and set aside.
When you push the toggle it will click on the relay as long as you hold the toggle.
Only use it no longer than 10 sec. Then start the engine.

I have mine set up this way for several years. I installed a fuse on the hot wire under the hood,
used one of the one`s with the rubber cap.

My signature shows 292 K miles, but have 324K now and runs pretty good.
For some reason can`t change the numbers when I go in there.


Charlie

 

ya it always runs rough on cold start even plugged in on a warm day otherwise it doesn’t start fast enough it takes about 30 seconds of cranking if it’s not plugged in but still runs bad until that cold idle switch gets hot and the pedal flings back. And idk what plugs he gave me but they were pretty much brand new. I’ve heard on here to only use those motor craft ones though.

 

It`s stamped on the plug the name brand, if you are getting old,
might need a magnifying glass.

Charlie