OK,2.8 gurus!Need some help,hunting idle etc 1991 2.8

[jtcfanof3]

Ok,so after racking my brain,testing the TPS testing the IAC(I think its OK)checking/unhooking the EGR etc Im lost.....
Symptoms:
Hard start cold or hot
Intermittent hunting of idle
When it does idle ok,under load it surges sometimes backfiring through the TB.
I have checked plugs (10,000 miles on them)seem ok,a bit leen maybe,no carbon build up,this leads me away from unburnt fuel.
I have checked timing,seems to be inline with correct setting.
AT closed throttle the TPS reads less then the 1.25V reading,but doesnt meen its not bad though.Checked cap/rotor seem ok,cleaned the contacts.
Again,happens sometimes and sometimes not,when it starts and idles fine it has no power underload and bucks/surges,but other times it "hunts" for the correct idle.
Now am I wrong in saying TPS/IAC/ICM?
Please help me out,this is my old truck that my nephew now owns,hes 16 and needs his wheels............LOL!
Thanks in advance everyone!

[jtcfanof3]

Pretty sure Ive narrowed it down now to the IAC.

[Hotwire]

Take a piece of wire and short out the two upper left terminals in the ALDL connector and turn the key on, but don't start the motor. Wait about 5 seconds then unhook the IAC. Fire the truck up and see if the idle is still hunting, this will eliminate the IAC as a problem.

If it is hunting, than it is working and something else is causing it to bring the idle up and down. Have you checked for vacuum leaks? That would be my next suggestion.

[jtcfanof3]

Ya,all over the motor lookig for leaks,even sprayed some stuff all around the intake,and no idle change.
I will try that trick tomorrow.
Its so spiratic,seems once its warmed up it will run better,but unti its warm it doesnt want to start,like IAC is stuck way open!
I meen it sat for a bit in the garage before I tokk it back across town,didnt want to start,bucked and galloped barely ran etc.....
I know the IAC is just a step switch basically but can be a cause of the hunting so thats what led me to thinks its the IAC.I tested the TPS and its dead on below the 1.25V at full close and goes up and down as the idle hunts.

[91sonomast]

check ur timing advance connector in the passenger floor behind the carpet

[jtcfanof3]

DId and its fine,just double checked timing again,its a bit advanced,but not much over the 10*btdc mark.

[Rustdaway]

Had the issue with my IAC, but it wasn't bad. Problem was the AC line had a hole in it and it was telling the ECU that the AC was on sometimes. Pain in the rear!!!

Not sure on the GM products how to disable the AC, but would try to just unplug it right behind the pulley. See if that helps.

Had the same issue with a 4.3 and it was the map sensor though.

[jtcfanof3]

IAC replaced today,runs great again!

[jtcfanof3]

CRAP,its back again,hard start and hunting idle,it ran fine yesterday,now today same thing!
TPS maybe???

[Hotwire]

I just had a very similar thing happen to my 88 jimmy. It would hardly idle, spray pattern @ the injectors looked fine until I opened the throttle and the spray got weaker and weaker. Turned out the rubber line that goes from the pump to the hard line that exits the tank had ruptured. It was spitting the fuel pressure back into the tank.

Take your timing light and use it to watch the spray pattern of the injectors. Watch to make sure the pattern stays in a nice fanned out spray.

Only other things I can think of are an intermittant connection in / around the distributor.

Have you shorted out the ALDL and checked to make sure you aren't getting any codes?

[jtcfanof3]

I swapped over my MAP sensor and TPS from my 91 305 V8 TBI,still nothing.
Swapped over the coil pack as well,nothing.
So tomorrow Im gonna pick up a cap and rotor for it,if that doesnt fix it,then Im gonna replace the ICM in the Dist.
I know how to test it,kinda complicated,but Ill give it a whirl anyway before I have him replace it.
It does not hunt for idle anymore,now its acting more like a bad cap/rotor.
I have rechecked plug gaps and wires etc etc....

[jtcfanof3]

PLEASE help me out on this guys!
Ive sprayed carb cleaner ALL over that motor,I can NOT for the life of me find any leaks!
Ive sprayed inside the cab etc........
Im running @14 inches Hg (Mercury) at idle on Vacuum at the TB so.......its not way under,normal is what 16-22??
I swear guys,Im gonna blow this damn 2.8 up!!!

[jtcfanof3]

gauge reading steady 17-22 in Hg indicates normal engine in good condition

gauge reading low (15-20 in Hg) but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking carburetor or manifold gasket.

gauge reading steady but dropping regularly indicates burnt valve or improper valve clearance

gauge reading slowly dropping to zero as engine speeds up indicates choked muffler.

gauge reading fluctuating between 15 and 20 in Hg at idle indicates stuck valve or ignition miss.

gauge reading drifting indicates improper carburetor adjustment or minor intake leak at carburetor or manifold.

gauge reading fluctuating as engine speed increases indicates weak valve springs, worn valve stem guides

<LI itxtvisited="1">gauge reading vibrating excessively at idle but steady as engine speeds up indicates worn valve guides.

gauge reading vibrating excessively at all speeds indicates leaky cylinder head gasket.




Iny opinons on the above?

[jtcfanof3]

Any chance it could be a weak fuel pump?
Its hard starting almost everytime,even if its shut off and then back on.
I checked my base timing,its correct 10* btdc,and once the wire is pluged back in its advanced about the correct amount.
UGHH!
I give up!

[jtcfanof3]

Maybe a cloged filter?
When you first start the engine you can drive as hard as you want for the first 10-15 seconds,then the stumble kicks in.
This is why I think its a weak pump,its strong at first then drops off.