I replaced the fuel pump and timing belt and now the engine races when it's cold. If I drive it before it warms up completely, when I let off on the gas and push in the clutch, the engine roars. I don't have a tach so I can't give exact numbers. Before the new parts were installed, there was no problem with the idle. It went a little high when it was cold (as it should) and then dropped down to normal when it got warm. I replaced the Idle control valve previously. How do I go about diagnosing this problem and why would replacing the timing belt and fuel pump cause this problem to occur?

 

So it only does it when Cold, and idles normal after 5 min warm up?

Might check the PCV Valve, it should be changed every 3 or 4 oil changes any way, PCV Valve is a "controlled" Vacuum leak
The PCV valve should be closed at idle, high vacuum, then opens as engine vacuum drops, acceleration
Since the PCV valve sucks in blow-bay gases AND oil vapor from the engine it will get a good(bad) coating of oil inside
When this oil gets cold it can hold the PCV valve open..........until engine/PCV Valve/oil coating warms up, then all is well

 

I changed the PCV valve a couple of months ago just because it hadn't ever been changed. Is there anything about replacing the fuel pump and timing belt that could have caused these symptoms? I don't see how it could but there's a lot I don't know and most of my knowledge comes from UTUBE when I need to know how to fix/replace something. By the way, I did replace the fuel pump myself but I took it to a garage for the timing belt because I didn't want to risk screwing up the timing and spending days or weeks trying to figure out what I did wrong.

 

No, changing fuel pump or replacing timing belt would have no direct relation to higher cold idle

But engine work requires removing and re-installing hoses and wires, so................

Not sure of the time frame
Did you do the fuel pump and then have the timing belt changed?
If so did you notice the high cold idle driving it to the shop?

 

I tested the fuel pump because my truck died while I was driving it and I had to have it towed home. I had very little fuel pressure so I changed the fuel pump. But it still wouldn't start so I had it towed to the garage where they replaced the timing belt (the belt was broken). It never ran between replacing the fuel pump and replacing the timing belt. I'll have to go under the hood and look if I can see anything amiss.
Another annoying problem is that several days after replacing the entire fuel pump assembly (which included a new sending unit for the gas gauge), the gas gauge went to empty and it's been sitting there ever since. It's not worth dropping the gas tank or removing the pickup box to fix it - I'm assuming that the new float had a slow leak and after a few days it filled with gas and sunk.