Troubleshooting WAY too many symptoms: TPS code, high idle, harsh engine braking, etc

Hi folks!

So I have been searching these forums for the last couple of days and gathering lots of good info re: my truck's issues, reading lots and asking a few questions here and there, but I just hit the "in over my head" mark and I figured I would just put it all out there and see if anyone has any ideas.

The details on my rig are over on in the column on the left hand side of the screen there

I am in the process of moving across the country, and stopping to visit friends and family and what not, so I have been driving A LOT in the last few weeks.

The first thing I noticed after driving nearly 1,000 miles, is that the engine braking on my rig has been getting increasingly harsh. Its always been fairly rough, but in the last few 100 miles or so, its become really jerky and sudden: immediately after removing my foot from the throttle pedal, she just pulls back hard and slows quickly. This happens in all gears except 5th, although sometimes in 5th there is some stuttering, which is also new.

Also, my MPGs have been getting steadily worse. A month ago I was getting an average of 23 mpg, now its more like 20. I record MPGs at every fill up. I have 9 months worth of data.

Next symptom, just a few days ago, the CEL goes on with two codes indicating TPS issues. I had the CEL reset, just to see if it would throw the same codes again. No codes yet.

And the last symptom, which has been with me since I bought the rig on day one, is a high idle. Cold idle is 2000 rpm, warm is 1250 rpm. The reason I have been ignoring this is because when I first took it to a mechanic 9 months ago, he said "that doesnt SOUND like 1200 rpms to me, its SOUNDS like a normal idle of 800 or so. I think there is something wrong with your tachometer, so I'd just ignore it". But now that all this other stuff is popping up, and I am reading more on forums about these issues, I am starting to worry about the idle again.

Incidentally, another thing I have noticed since day one is that the engine warms up really quickly. Just two minutes of driving and I am at normal operating temp.

Here is what I have done so far:

I removed and cleaned the throttle body and and IAC valve. Both were dirty but moving freely, and I observed no change in performance, good or bad (except for a wicked high idle right away, but that went away after 25 miles or so of driving, once the computer adjusted).

With the engine warmed up, and running, and idling at 1250 rpm, I unplugged the TPS. The idle instantly shot up to a smooth 2000 rpm. I plugged the TPS back in and it lowered back down to 1250 rpm. As I understand it, this means the TPS is functioning properly, right?

Next, with the engine still running and warm and idling at 1250 rpm, I unplugged the IAC valve. At first nothing happened, but then after about 5 seconds the rpms shot up to a smooth 2000. Also, when i got back in the cab, the CEL was on. This is one thing I am not sure of: some people say killing your IAC should cause the engine to stall: if not, you might have a vacuum leak somewhere. others say "not necessarily. engine could still run with IAC valve unplugged from power". What do you say?

Last, I took some of the slack out of the throttle cable. No change observed, good or bad.

So right now I am thinking vacuum leak somewhere, causing the engine to run lean, but I figured I would check with you guys. Mostly I am wondering what you think about the IAC valve's behavior when I unplugged it. Also, I am wondering what y'all think the meaning of the harsh engine braking is.

Do you think the next step is to take it to a mechanic and have them check for vacuum leaks? I don't really want to spray carb cleaner on a warm engine, and I feel like blowing cigar smoke into the intake hose isnt going to be all that reliable, so...

I guess since I got the TPS codes, I could just go out and buy a new TPS and see if that clears things up, but I'd rather diagnose the problem than just throw parts at it.

Thanks for all your advice, guys!

 

Disconnecting sensors and seeing the effect is a pretty unreliable way to test things. Once the computer knows a sensor is disconnected, it goes into a fail-safe mode. It adjusts timing and fuel so the car will at least be drivable. So you can't tell much by how it's running at that point.

The IAC valve has a spring that holds it half open/half closed when no electric current is applied, so it's seems reasonable that the idle would go up when you disconnected it.

Download a copy of the factory service manual for your year here. It will show how to test the sensors and IAC with a multimeter or by applying voltage to them. It's all pretty basic stuff if you know how to use a multimeter. It's better to do this, than throw parts at it.