1993 K1500 TBI stumbling/surging

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Well a few months back I picked up a 93 GMC 1500 2 wheel drive rez truck from a friend. 370,600km on the clock and V8. Its got a few problems but otherwise is a decent truck for what I need it for. I replaced the brake switch and got the TCC locking up again but have to chase a low speed surge/stumbling problem. The hose that routes the PCV to the throttle body was rotten so I have that plugged for now. But the main issue is this: once the engine is up to temp it stumbles from idle until around 1500rpm and then it levels out and drives fine on the highway and pulls great through the rev range. It idles fine but once you tip in that is where it stumbles. It never dies.

When it is cold it starts and runs fine. Because of this I was thinking oxygen sensor first. Thinking closed/open loop operation.

Any thoughts?

Thanks!
 
Throttle position sensor, check it with a voltmeter. Also check that the tubing from the MAP sensor to the intake is intact.
 
It's acting like the accelerator pump is not squirting when the engine is coming off idle, but this engine is throttle body injected; no carburetor. Hmm...
 
Originally Posted By: StevieC
Could also be a coil / wires breaking down or leaking spark at specific times. But check the TPS first.
+1
 
Disconnect the EGR and plug any tubing you disconnected in the process. See if this helps. If so, I can be the EGR, cracked tubing, or the control solenoid.

Carbon may also prevent the EGR from fully seating. After doing the above, I'd also manually manipulate the EGR diaphragm through the slots to check if it's stuck.
 
Spent A LOT of time attempting to diagnose a friend's '89 K1500.
The engine sputtered badly. No real telling if higher revs changed anything though there was some smoothing at speed.
The symptoms pointed to fuel delivery and/or spark management.

THE FIX: Turns out there was a built in tap off on the battery cable (big red one) coming from the clamp. It powered the nascent fuel controller (the flat black 4" X 5" thing held by a bracket right near the throttle body-don't know about '93). The lead swaging of the clamp was full of lead oxide. A new cable fixed all our ills.

FYI: The "computer part for the ignition" was housed in a compartment which was a bulge. Half of this bulge was the back of the glove compartment and the other half reached into the engine bay. The firewall was bulged to accommodate it.

Hope tis helps.
 
this one is a classic failure.pcv system blocked caused the blowby to enter the tbi through the vent.
rots the fpr spring and drops fuel pressure.
the tbi baseplate passages plug with crud.
clean it up and rebuild the fpr first.
do not plug off the pcv!
fix it.
also make sure the sensor ground cluster at the water outlet is clean and tight.
these 2 things account for the lions share of problems.
 
that black module is the esc.
fuel management is in the ecm.
loss of power via that battery tap will indeed cause lots of issues.
Originally Posted By: Kira
Spent A LOT of time attempting to diagnose a friend's '89 K1500.
The engine sputtered badly. No real telling if higher revs changed anything though there was some smoothing at speed.
The symptoms pointed to fuel delivery and/or spark management.

THE FIX: Turns out there was a built in tap off on the battery cable (big red one) coming from the clamp. It powered the nascent fuel controller (the flat black 4" X 5" thing held by a bracket right near the throttle body-don't know about '93). The lead swaging of the clamp was full of lead oxide. A new cable fixed all our ills.

FYI: The "computer part for the ignition" was housed in a compartment which was a bulge. Half of this bulge was the back of the glove compartment and the other half reached into the engine bay. The firewall was bulged to accommodate it.

Hope tis helps.
 
Originally Posted By: kc8adu
this one is a classic failure.pcv system blocked caused the blowby to enter the tbi through the vent.
rots the fpr spring and drops fuel pressure.
the tbi baseplate passages plug with crud.
clean it up and rebuild the fpr first.
do not plug off the pcv!
fix it.
also make sure the sensor ground cluster at the water outlet is clean and tight.
these 2 things account for the lions share of problems.


I agree with this. My 1990 K1500 has the 350TBI....and this fixed many of my problems as well.

The PCV on these things really tends to plus in the TB. I'd go to NAPA and get a TB rebuild kit. Take the TB off, clean it all up, replace the FPR diaphram and all the seals and gaskets and make sure the PCV port is cleaned really well. This shouldn't take you too long, it's kind of fun because it's all so easy, and you'll know from now on you don't have a fuel problem.
 
hopefully all the kits have the spring now.
many in the past did not.most common failure part and not in the kit.stupid.
 
Originally Posted By: kc8adu
hopefully all the kits have the spring now.
many in the past did not.most common failure part and not in the kit.stupid.


Mine didn't have a spring...this was the kit from Napa about 4-5 years ago.

I cleaned up the spring the best I could. I then put some light grease on it to prevent it from rusting. Since the spring "might" have been "tired" and lost some of it's function, I put a single US $0.25 Quarter in the throttle body to act as a spacer for the spring to give it a bit more force onto the FPR.
 
the $0.25 mod is a common way to boost fuel pressure a bit.
as for the spring most are in at least 2 pieces when i see them.the blowby backing up due to no vacuum on pcv blows right on it and rots it fast.
drive too long like this and the engine will sludge up.
 
Originally Posted By: kc8adu
the $0.25 mod is a common way to boost fuel pressure a bit.
as for the spring most are in at least 2 pieces when i see them.the blowby backing up due to no vacuum on pcv blows right on it and rots it fast.
drive too long like this and the engine will sludge up.


kc8adu: No doubt. When I bought my truck, it was a sludge MONSTER. I took the VC's off and scooped the crusty nasty crud out. Several times I filled the engine with kerosene and sometimes a mix of kerosene and toluene, disconnected the dizzy cap and would would crank it over for 15-20 second burst several times through out the day for several days. Drain and refill.

I bought my K1500 with 150k miles for $1800. I've put over 60k miles on her. She's still running decent.
 
Thanks guys for the info. I have a new line for the PCV on and will take it for a rip today to test. Then onto further vacuum leak testing and then will check the egr, iac and then if that does not cure it, will dig into the tbi.

Now that the brake switch is working, which causes the TCC to work as designed, the stumble was much more noticeable because the revs are so much lower while cruising.
 
Just some followup. I put in a new hose to connect the pcv and the throttle body. Cleaned up the stumble a bit but still very prevalent. More of a surge now while cruising around under that 1500rpm. Tested for vacuum leaks with carb cleaner on all the connections and came up clean. So once its cool and get a chance then I will rip into the egr, iac etc. What oxygen sensor do people recommend?
 
I'm betting if you rebuild the TBI....you'll be good.

Another common leak is the TBI to intake gasket leaking. Your TBI rebuild kit should come with a new gasket for these mating surfaces as well.
 
So I found some more time to look into this. I have been researching this more and how to test the sensors and various systems to find the fault. I was never big into electronics/electrical systems but decided that now is better then never to learn. I tested my ECT for resistance. Approx -10C ambient and was measuring 17 Kilo ohms and change, which from what I can find is accurate. Ran the truck to work, 40km on the highway at 100+km/h. No IR gun but she was up to temp and had ample heat. Resistance was now 217 ohms. Which again corresponds to around the mid 90's C. I would say that the ECT is working.

next up is to test the voltage signal in the TPS and the single wire O2 sensor.

This is a whole new world of diagnostics for me and its cool! Very impressed with the new multimeter. It helped me trouble shoot the parasytic drain in my Trailblazer in mere minutes.
 
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