cold start rough, hot start fine, mileage poor

[Pete]

Guys here's a question for you... '91 XLT, manual trans, A/C. On cold starting, it will start right up but runs REAL rough for about 10 seconds, then it clears up and idles and runs normally. Strong smell of raw fuel in the exhaust when it does this. My gas mileage is down about 25%, too. Hot starts are OK. Any ideas? I have not checked for codes yet; will do that this weekend. It has a new oxygen sensor and the fuel filter is about 6 months old. Truck seems to run fine at all other times: idle, off-idle, hot or cold, etc., all fine.

Thanks ! ! !
Pete

 

[MrShorty]

I might start by cleaning the IAC valve.

 

[Pete]

Thanks for the tip. Is the IAC also known as the idle air bypass valve?

 

[TPLYNCH]

Yes. Also, may want to replace your Fuel Pressure Regulator and check and make sure your intake manifold bolts are properly tightened.

 

[Pete]

Thanks TP I'll check them. I have a list of things going to check.

Dang regulators got expensive... last one of these I did was forty bucks for a Bronco II a couple years ago... this one is $112!!! Yikes!

 

[Opera House]

The 91 regulator is wierd

and that is probably why it is expensive. Think it has a pigtail return line attached to it. A 92 is only about $60. Pull off the vacuum line on the bottom and see if there is gas coming out. If so, that's what you have to spend. With a little efort, a 92 could probably be adapted.

 

[Alec]

I also agree the FPR is the likely problem.