It's in a 98 extended cab, auto, 2x4. There is a few things going on with it, I don't know if they are related or not but I will list the problems I'm having.
First off when I start it in the morning it always starts right up, but it runs a little bit rough, I go to work (2 mile drive) then for lunch I go home, it starts fine again, but when I start it to go back to work after lunch, 3 out of 4 times it's hard to start. I have to put my foot on the gas to get it running and some times I have to rev it up a few times or it will stall out, but once it runs for a minute or two then it is ok. Keep in mind it always runs a little bit rough. I was thinking maybe it could be the fuel pump going ? I did a tune up with cap, rotor, plugs, and wires and that helped the rough running some, but didn't fix it and did nothing for the stalling out problem.
Another problem I have is at speeds above 55 I notice that some times the rpms rev up just a little for a second or two and then back to were it was, it does not always do that but when it does it seems to do it every 1-2 minutes. I thought possibly a intake leak but I couldn't find one.
Another problem I have had is the cruse control works some times and not others, and the ABS and red Break light comes on together some times, but not ofter. I did some searching and found that could be from a speed sensor in the rear end, I was wondering also if maybe this could all be related to a computer problem.
My biggest concern is the stalling out and rough idol, if I could fix that I would be real happy, I did hook it up to the computer and it isn't putting out any codes.

 

I had the same problems, changed everything i could and it ended up being the computer. save yourself the headache and have it tested.

 

If the computer doesn't work than i have another idea, i while back i had kind of the same problem, ruff starts, not wanting to idle, and stalling out, did some looking around and come to find, there was a leak in the intake, not a crack in the intake though, it was a vacuum port, since i don't have A/C I have two little ports on the left and right of my intake with little rubber stoppers on them, one of them had fallen of due to expanding. so put it back on, with a zip tie and bam, its work just fine after that. So if would check that first before buying a computer.

 

Check the Throttle Position Sensor. Not starting unless you floor the gas pedal can be a sign the TPS is bad. So is the problem with the RPM jumping around at speed. The Haynes Book for your truck will tell you how to check the TPS with a volt meter, it is not hard to do. Also remove the Idle Air Control Valve and clean the black crusty crud off the end of it. This could be the reason for the rough idle. Handle it very carefully and don't drop it or you will have to replace it. Use a shop rag soaked in carb cleaner to clean it and the orifice where it mounts on the back of the throttle body. Put a little clean motor oil on its o-ring when you reinstall it. See if that helps.

Jimmy

 

Ok I will try testing the TPS and cleaning the IAC, isn't the IAC hard to get too on these 3.9s ? I will also recheck for a vac or intake leak, and if none of that fixes it I will have the computer checked out, will they check it at auto zone or do I need to find a shop that will do it ?? Thanks for the ideas guys.

 

The IAC is on the back side of the throttle body, held in with two Torx T-20 screws. It is much easier to remove it if you just remove the whole throttle body. Disconnect the throttle cable and cruise control cable if you have one, remove the vacuum line from the front of the TB, unplug the MAP sensor on the front, the TPS on the side and the IAC from the rear. Then remove the 4 bolts holding the TB in place on top of the manifold then you can easily remove and clean the IAC and its mounting orifice. You can also remove all 3 sensors and clean the TB itself and lube the pivot points and linkages on it. This is also a good time to look down into the manifold to see if your intake manifold belly pan gasket is leaking. I personally don't know if Autozone can scan the PCM with their code readers. If the PCM is bad I would think it would throw a code for it. Get a new TB gasket to use when you reinstall the TB back on the manifold. Gasket only costs about $1.50 at any parts store.

Jimmy

 

I love this site,very informative thread just about everywhere I turn here

 

Thanks Jimmy, I am going to tear into this thing today. I am going to get an intake gasket and putt it off I have wondered if possibly the problem could be caused by an intake leak, so pulling it off and going through it, cleaning and putting in a new gasket will take that off the list of possible problems anyway. Thanks for the directions on how to get it off, doesn't sound to bad so we shall see.

Yeah I like this forum a lot too, lots of helpful stuff on here, and a lot of good people.

 

While you have the TB off, take a look down into the manifold. A very light dusting of oil residue is normal, it's from the PCV blow-by. If you see oil pooling in the bottom of the manifold, the belly pan gasket is bad. A bad belly pan gasket can cause a rough idle, loss of MPG, loss of power and increased oil consumption.

You can get a vacuum gauge at Harbor Freight Tools:

http://www.harborfreight.com/fuel-pu...ter-93547.html

On the passenger side of the manifold there is a vacuum port there with a black rubber cap over it. Remove the cap and connect the vacuum gauge. On a warm idle you should have 20 to 23 inches of vacuum. If it's less than that or if the needle fluctuates around or drops then slowly increases again, you could have a bad belly pan gasket or some other vacuum leak, or possibly worn rings or bad valves.

You can also have the engine on a warm idle, pinch the pcv hose shut with pliers, you will hear the PCV click shut, remove the oil filler cap and feel across the oil fill opening with your hand. If you feel suction there is a vacuum leak some where.

If you are not using any oil, the belly pan gasket may be OK. Check the TPS, with the engine off but key in ignition on position I think on the middle wire it reads .5 volts and the voltage should increrase slow and steady to about 4.5 volts as you open the throttle by hand. Anything different and the TPS is bad. There is a mod you can do to set the TPS voltage at .75, some say it gives better throttle response. I have never tried it myself. Sometimes it can be hard to get a good diagnosis on the TPS, may be better just to replace it. Same with the IAC, if after cleaning it you still have a rough idle maybe try a new one. I clean my own IAC every 20,000 miles and my original one is still good at 142,300 miles now.

Hope that helps. Let us know how it turns out.

Jimmy

 

Fixed !!! Thanks everyone for the help.

I started out by checking the engine for vac leaks again, but I took my time to check out all the lines and connections for cracks or dry rotted rubber this time. I did find some lines that were a little worn but nothing was leaking.

So then I started checking out the sensors on the TB and I was getting ready to pull it off but I first checked the connections on the TPS and the other sensor on the front (mass air flow maybe ?) anyway the TPS looked a little dirty so i took it off and cleaned it up, and cleaned up the connector tabs and put it all back together and bam it works again !!!!! I plan to replace the TPS and I am still thinking about pulling the TB off to clean it up and clean the ICA and maybe replace it too since its only 30 bucks at autozone. But I'm happy as can be yay my truck is running good woot woot !!!



Thanks again for your help guys.