OK 88 chevy truck 3/4 ton
350 TBI

Engine is hard to start without opening the throttle slightly, when it starts I let the throttle close, it sputters a bit then smoothes out but it idles extremly high (2500-3500rpm) When I give it gas it will do 1 of 2 things, spit and sputter or just die. I first pulled trouble codes and got a code for EGR and a code saying there was no signal from the knock sensor. I suspected a bad vacuum leak. Here is what has been done so far

replaced the following parts with new ones: EGR valve and gasket, Knock Sensor, IAC, Throttle Body Gasket, plugs, plug wires, cap , rotor, installed MSD Pro-Billet Dist with new module, coil, all vacuum lines have been replaced.

Still have the same problem So I checked the intake gaskets for leaks, seem to be OK.
Now I suspected a Burnt/broken valve or seat
Did a compression test all cylnders are good so that rules out valves or head gaskets or rings

All spark plugs look normal after running a while

It really acts like it is a massive vacuum leak, but where?
How likely is it that the intake gaskets could be leaking on the underside in the lifter valley?

I have also tested my MAP and TPS, both are good

Is it possible a bad fuel pump would make an engine idle high?
I know the throttle blades are completely closed so It has to be getting air from somewhere.

Could my ECM or ESC be the culprit? If so how do I test them?

Every sensor on the motor is new or have been tested so the only other guesses I have are:
Intake leaking on lifter valley side.
Bad fuel pump?
ECM.
ESC.
Has anyone else had a similar problem?
Does anyone else have any suggestions.
Thanks
Jimmy

 

Check the TPS. That gave my friend all sorts of problems and he did nearly the same thing as you have. Also, (a shot in the dark, but worth checking out), your O2 sensor. I hope it helps you.

 

easiest way to check if it is the ecm(i am warning everyone, this is far less than scientific) is to take it out and do waht ever you can to get it idling, then tap on the box with a screwdriver handle or something. usually when the ecm is flaking out/dying it will go all nuts when it is tapped on. a good box will idle just fine and nto act any different.
they way to see if it is the fuel pump woudl be to check if the pressure is stable, if not it is bad.
i don't know why it woudl try to idle real high unless the ecm is flaking out somehow, the only way th ecar can raise it's own idle is withthe IAC, and you said you had replaced that.
did you have any other codes besides the EGR?

later
tim

 

Thanks everyone for the replys. I had already tested theTPS, it is fine. The o2 sensor was replaced also so thats not it. I will try tapping on the ecm. I just pulled the intake to replace the gaskets just to be sure so I will reassemble tommorow and give it another try. What is the best sealant to use on intake bolts? No other codes were stored except a code for the knock sensor. Book says that the ESC is not recieving a signal from the sensor so I replaced it too, although I do not see how that would cause my prob.
Thanks
Jim:

 

His TPS tested fine too. It was bad.

 

I had the exact same problem. I didn't replace anything to get it fixed. I just bought the re-build kit from Autozone and cleaned the **** out of it. Took about 2 hours. It may be a fluke or i guess it coulda been the pressure regulator.

 

high idle usually leads me to suspect a vacuum leak...

 

I had a problem almost exactly like that on a 305 TBI Silvarado. Turned out to be a bad CTS (you gotta love having a scanner). It was reading way wrong so it was hard to start without throttling it on crank, then it idled WAY high once it started (after a bit of sputtering).

 

Throttle body gaskets are really bad about leaking on those pickups so check that. I replaced one the other day that was causing a high idle, it also had a bad EGR and a bad fuel pump that was causing it to surge. Good luck!

 

Jim Did you figure it out yet?

 

My 350 TBI is doing the same thing. I start it up, it'll sputter then even out. As soon as you touch the trottle, it misses all over the place. Under load it misses like crazy. Other times I start it, it'll idle high, around 1600 rpm's. So far I've changed the TPS, MAP, O2, EGR solenoid, cap & rotor. I think the computer is crapping out. I get 34 & 44 codes too. And when I start it, my windshield wipers will sweep once and stop, even with the switch off.

 

hmm... just throwing ideas here.. but my buddy's 93 tbi pickup has been going through hell.. he replaced just about every sensor and even the pickup coil and was still having weird problems... on the week-end we checked the reluctor on the distrubutor and sure enough it's all full of play.. you hold the rotor and you can move the reluctor 1/2 turn either way.

 

My vote is CTS, sounds like your already in closed loop, start-up requires enrichment which is open loop. Try disconnecting the ect it will go into a fixed mode. THIS IS NOT A REPAIR THOUGH, your just looking for the condition to become less rough.

 

Thanks to all for all the replys. It turned out to be a bad CTS. although after doing a compression test I decided to go ahead and replace the motor. Its seems as though I have a cylinder on its way out. 330 HO is getting picked up tommorow. Does anyone know what my stock motor is rated at as far as power and torque go? I think its about 200 hp but Im not real sure on the torque.
THANKS AGAIN EVERYONE FOR YOUR HELP. IT IS APPRICIATED.
Jim

 

Are you sure it's rings and not valve seats? How many miles were on the truck? If it is 8600gvw or higher it is 210hp. If below it is about 165hp. I am putting a new short block in my 93 Burban. Doing the vortec heads and new gm intake. If you want a good engine place try KP Engines out of Kent, WA. $671.00 for a short block 350 2-bolt main.