Hi,

I have a 1993 F350 with a 7.5L. I rebuild the engine about 3500 miles ago. It is my first engine rebuild. So far there are no issues, every thing is working perfectly.... except for the cold start and the rough idle that follows. I have removed the catalytic converter ( no emissions test in my area) this made it run a lot better. I have also replaced the EGR sensor which helped with the rough idle when cold.

But it still isn't perfect. It still idles rough when cold and smooth when warmed up.

There is noticeable black smoke when its cold and no smoke when warm. I assume this means its running rich. The O2 sensor is new. The vacuum lines are good. CK engine light is not on.

Any one have thoughts on how to fix this.

Thanks

 

Have you checked for stored DTCs? The check engine light may not be on but there may still be trouble codes stored in the EEC. That's an easy place to start.

Someone please correct me if I'm off base here but could this also be a symptom of a sticking EGR valve? You mentioned you replaced the solenoid but did you replace the valve too?

 

Have you checked fuel psi?

It could be as simple as a leaking injector(s). Pulling a plug or plugs could narrow down the problem if any appear black or wet when you allow it to sit overnight or more than several hours.

 

I tried to check DTCs, but wasn't having any luck with the figuring out the flasing lights on my test light. I'll try that again. I did not change out the EGR valve because it's pretty expensive and I wanted to be sure this was the problem before I spent the money.

Thanks

 

I did not check the fuel pressure, although I need to do this. I'll rent a gauge and check it out. I'll check the plugs also.

Why would leaking injectors cause rough running only when cold?

Thanks

 

Mine was doing the same. New motor 300 miles ago and still did it. I replaced both fuel pumps and the fuel pressure regulator. Starts like new and idles smooth.

I started it and ran it after each pump replacement and it was still a little rough. I changed the regulator and that took care of it. I would say to start with the regulator.

 

Need to verify STATIC fuel psi by grounding pin 6 on the DLC connector. Check for leak down.

Check Running fuel psi after establishing Static psi reading.

 

I just replaced the fuel pressure regulator. This helped a little with the rough idle when cold, but it's still not perfect. So far I have a new fuel filter, front fuel pump, vacuum lines are good, new FP regulator, no Cat, new IAC ( I thought I had replaced the EGR solenoid, but it's the IAC) All these things helped just a little in smoothing out the cold idle, but still it's there. Should I still check the fuel pressure? considering I have most of the fuel system replaced. The injectors are old. I'll try again to get the codes.
Any thing else.

Thanks

 

Ok, gonna be blunt here. Why keep throwing parts and $$$'s at it, if your not going to use the diagnostic tools we have mentioned to use?

You said everything is new except for the 20+ year old injectors. Just rent, borrow, or buy a fuel psi tester and then you will Know what you have and don't have. Have you checked the plugs after it has sat for several hours to see if any are wet with fuel? That would also point to leaking injector(s).

 

All that's next.

All the replaced parts ( fuel pump, Cat, IAC) were replaced months ago because they were bad. FP regulator is today. I don't feel that the other replaced parts were money wasted. Every new part made it run just a little better. The Cat delete was the biggest improvement. I don't understand why a leaking injector will affect only the cold idle and perfect when warm. What does static fuel PSI mean? Is this the PSI with the engine off and pump running? What should the pressures be when idling?

Thanks again.

 

OK, I know it's been a little while since I followed up here, but here are the fuel pressure results.
Key on, engine off, pump running = 43 PSI
engine running = 37 PSI
There seems to be a gradual leak down of about 3-4 PSI within several minutes of turning off the pump.
I also just noticed that when it's hot out > than 80 degrees, there is no rough cold idle.

 

You may have a bad coolant temp sensor. This device tells the ECM how rich (or lean) to make the fuel mixture. It is basically a thermistor. They do go bad occasionally. It could be when exposed to cooler temps the resistance across it is much higher than it should be - causing the rich condition and black smoke. You might also check the wiring leading to it.

Good luck.

 

Does this mean the pressures are good? What about the decrease in PSI over time?

It's a new coolant temp sensor, but that doesn't mean it's can't be bad. It's cheap enough to try it with a new one.
Thanks.

 

Which one did you replace? There are two, one each for the dash gauge and computer.

 

Don't recall exactly but I bet it was both. I had the intake professionally cleaned so I doubt I put in the old sensors. I'll just replace the computer one for now and see how it goes.
Thanks