My son is now the proud owner of my old 89 Chevy 2500 4X4. It has a TH400 and a 350, which have both been rebuilt. It has ~ 80k on it and has always ran very strong with zero problems... Until recently.

A couple of weeks ago he called and told me his truck was running very poorly and that it had run fine the day before.

It started out looking like a fuel problem but no so sure now. I can get it to run and idle fair IF I unplug the MAP sensor. If unplug the MAP and put it into gear, it will die.
BUT, if I leave the MAP unplugged (Vacuum line only) AND disconnect the TPS, it will continue to idle once I put it in gear.

I have replaced/inspected so far:

Fuel filter
TPS sensor
IAC
Distributor(complete including rotor and cap)
MAP sensor tested (I may have diagnosed incorrectly, however)
Plugs and wires
EGR is working

As noted, I can get it to run smoothly in park if I unhook the vacuum to the MAP but if I put it in gear it will bog down and die.

AND, if I unplug the MAP AND disconnect the TPS, I can put it in gear and it will continue to run. To the point I am pretty sure that I could drive it. But if I plug either of them back in, it will die.

I could use any input on this one.

Thanks in advance.

 

Try replacing the coolant sensor.

 

But make sure you get the right one. There are 2, one for the gauge/ light and one for the computer the one for the computer is the one you want to change. But......this is a "cascade system" and my gut tells me that you need to test the tps. I will have to find the procedure and post the link...unless someone else finds it first.

 

Here try this.

 

Thank you for the link. I will check it out right away. When you say that it is a cascading system are you basically saying that any weak link can take out the entire system and cause the same type of symptoms regardless of the particular part?

Ignorance here but why, do you think it is when I unplug the MAP vacuum and disconnect the TPS and the truck will continue to run when I put in gear? Would it be safe to see how it drives like this on a short run?

Also, I posted the stuff below in a different thread. Any thoughts?


"Do you guys think that this could be a fuel pump problem? The fact that I can get it to run smooth and rev in park kind of kept me awayy from the pump but not so sure of anything now. I need to get a couple of fitting to adapt my fuel pressure gauge the truck. Should it just go inline at the fuel filter area? If so, do you turn on the key and see what it pressures up to or should I tee te line so it can be read while running? I do LS1's and not too familiar with these engines.

Last, when you turn the key on after the truck has sat for a day or so, how long should the fuel pump run to pressure up the line??"

 

you understand the cascading system right and when you unplug the vac line from the map you give it a vac leak which makes it idle higher than normal. If you drive it like this you will set all kinds of codes but will not hurt anything....im willing to bet you have a bad tps or map sensor....the bad coolant sensor would set a code in the computer as SHOULD a bad tps or map....the code you get (if any) should be the failed part unless you get more than one code (which Im pretty sure you will) then you have to find the "begining" of the "chain" and fix that part. if you follow what Im saying.

 

Thanks.

I did drive it a little bit with the MAP and TPS unplugged. I was ok as long as I gave almost zero throttle. If I did, it would just stumble, as expected.

I pulled the codes after the drive and got 15, 22, 33 and 34. All codes for parts I have replaced EXCEPT the coolant sensor.

I will pick up a new coolant sensor this week and try it.

Also, it took a full 15 seconds from the time I turned the key on (without cranking) for the fuel pump to pressure up and quit running. Thoughts on this? would guess the pump is about gone.

Thanks again.
-Robert

 

When that happens the best thing to do is try next time with a fuel guage to be sure.

 

Could very well be the fuel pump. There was just another thread from a guy that sounds very similar. I told him the same thing...my brothers truck was doing the same thing about 8 months ago and it ended up being the pump. It would run good one day and then the next barely make it to his destination. Luckily it completely went out right in his driveway and would no longer start. We lifted the bed and installed a pump and it has ran perfect for the last 8 months. He even cheaped out and put an aftermarket pump in it. It's been working though.