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1990 Suburban TBI idle issue

vrtillman

vrtillman

Member
Joined
Nov 7, 2022
Member Number
5736
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15
Loc
Denver, Co
I've got an issue with my 90 burb. Since I bought it a month or so ago, its had this low/erratic idle issue. It used to actually die at stop lights, but after swapping in a new fuel pump, it just idles low (350-400 rpm). It's most apparent when the engine is up to temp, but it will happen on cold days. I've figured out that the tbi injector pod hosing is cracked (ordered a replacement off fleabay) and it has mysteriously low vacuum, but i don't suspect ists from the cracked housing (cracked at the base so no excess fuel seems to be leaking out). So far I haven't found any vacuum leaks and my last shot is changing intake manifold gaskets (hopefully this weekend), but if i's not that, could there be some mystery cam in there causing low idle vac?

Details so far:
  • 1990 gmc v1500, tbi 5.7/700r4, 5.38 gears, ord 6" lift w/ shackle flip, 35" tires
  • unknown engine history (apparently it was bored .030 over at some point which makes me think maybe someone put a goofy cam in there and the stock ecu doesn't like it)
  • Replaced: plugs, wires, cap, rotor, EGR valve, fuel pump, fuel filter, coolant temp sensor, tbi base gasket, harmonic balancer (slipped), o2 sensor, IAC
  • Swapped for diagnostics: map sensor, ignition control module
  • IAC relearn done
  • confirmed timing was at 0 deg with ref unplugged
  • plugs are gapped at .035
  • re-tightened header flange bolts
  • verified there are no dead cam lobes
  • to be replaced: manifold gaskets, tps (voltage reading at wot was way too low), TBI injector pod housing
  • confirmed no vacuum hoses were broken (other than the ac/cruise hoses, but those are capped off at manifold for time being)
  • everything is hooked up according to the emissions sticker (minus cats for right now)
  • I need to recheck codes in a bit because i had the battery disconnected for some stereo wiring this weekend, but the only one that popped up was code 34 (MAP Sensor Circuit Error (Signal Low Indicating High Vacuum), but I'm assuming that's a result of the low vacuum and not what's causing it.
Am I wrong to assume there's an aftermarket cam in the engine making the ecu upset if the new manifold gaskets don't fix my vacuum issue?
 
What's the TPS read at idle? Any signs of someone jacking with the idle stop screw? GM TBI is real picky about the TPS.
 
M92PV4U said:
What transmission does it have?
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700r4
TrikeKid said:
What's the TPS read at idle? Any signs of someone jacking with the idle stop screw? GM TBI is real picky about the TPS.
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0.56v and 2.89v at WOT. I've read WOT needs to be in the 4.5-5v range. And no, I messed with it initially to try and implement a bandaid fix, but I kept track of what i changed and its back at the stock setting now
 
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What does the spray pattern look like on the injectors? I usually use an adjustable timing light to see the pattern.

Did you verify timing off the balancer, or off the valves? Could the balancer have slipped, you try advancing the timing just to see if it helped?

if you hold the throttle to a normal idle range, how's the vacuum? 350rpm ain't going to pull squat for vacuum.

You can spray some brake/carb clean around the base, at your cracked throttle body, to see if that's an actual vacuum leak.


Most TPS sensors have two ranges. Some will be from 0-2.5V and 0-5V, and some use a 0-5V and a 5-0V or 2.5-0. It's been a while since ive messed with a TBI one though, so maybe it only has the one range.
 
Dad's jeep runs an 87 350 tbi in his jeep with the same kinda problem.

You let out on the clutch slightly and it dies unless you catch it with the throttle. Hard to describe. Idles down low and just gives up.

We changed every sensor without luck and have a theory it's in the distributor and is maybe worn wobbly.

For now he burns up starters
 
BustedCrank said:
What does the spray pattern look like on the injectors? I usually use an adjustable timing light to see the pattern.

Did you verify timing off the balancer, or off the valves? Could the balancer have slipped, you try advancing the timing just to see if it helped?

if you hold the throttle to a normal idle range, how's the vacuum? 350rpm ain't going to pull squat for vacuum.

You can spray some brake/carb clean around the base, at your cracked throttle body, to see if that's an actual vacuum leak.


Most TPS sensors have two ranges. Some will be from 0-2.5V and 0-5V, and some use a 0-5V and a 5-0V or 2.5-0. It's been a while since ive messed with a TBI one though, so maybe it only has the one range.
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Spray looks even between the two. I just replaced the harmonic balancer because the one that was on there slipped. I can advance the timing, but i believe the computer tries to correct it if i have the reference plugged in.
Vacuum is still 15-16 if i hold it at like 700 rpm.
But I've sprayed carb cleaner all around it and nothing

Now I'm been thinking about what M92PV4U said and maybe the diaphragm in my brake booster is busted. I'm going to look into that first
 
Prob not what you need. I boughtChevy same as yours, same issues. Went crazy checking sensors. Mine turned out to be the computer.
 
After you check your booster out, unplug the timing reference wire, and advance it a little. See if it helps your idle and vacuum. I'd move on to checking compression and adjusting valves. Maybe even double check the firing order.
 
Well, update for today: I blocked off pcv and brake booster and..... no change. But now it has a fun new problem! I replaced the tps sensor and that effed up the whole truck. Now it idles too high (1400 in park and 900 in gear) and with key on/engine off, TPS sensor reads 0 v. And because I know someone will ask, I tried using the old sensor.... still 0 volts. Not sure why it just decided to shit the bed, but I'm thinking a new ecu is in order.

But I'll check valves soon. The PO of this truck is shaping up to be a real idiot with all the bullshit I'm discovering. He put on these goofy ass shorty headers that block access to just about all of the plugs so comp test is kind of out of the question since I don't have a way to remove the headers when the engine is hot. Oh, well....
 
vrtillman said:
I replaced the tps sensor and that effed up the whole truck. Now it idles too high (1400 in park and 900 in gear) and with key on/engine off, TPS sensor reads 0 v. And because I know someone will ask, I tried using the old sensor.... still 0 volts. Not sure why it just decided to shit the bed, but I'm thinking a new ecu is in order.

He put on these goofy ass shorty headers that block access to just about all of the plugs so comp test is kind of out of the question since I don't have a way to remove the headers when the engine is hot. Oh, well....
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You need to set the TPS to a baseline voltage, back probe and verify its at .28V or whatever is specified by GM at idle. Verify you have voltage to the map sensor. Check continuity through the pins for 5v, ground and, reference signal. Best to verify as many sensors as you can from connector to ecm. Since you had the map code and now tps is low I suspect you have a bad harness or poor grounds. The harness being 32 years old, and a shifty PO...chances are, there are some hack wire repairs, dry rot or, possible rub though. With you fucking around, the wires being moved is sometimes all it takes to break a wire or loosen a bad terminal. Heat also adds resistance so that may explain your warm condition. The computer not seeing MAP/TPS could be defaulting to a limp mode to run the engine at a higher IAC % but, I have seen debris fall in and hold the IAC passage open. Check that too. Map sensor is probably the most important for the ECM, it controls everything based off the table. You need to know what its seeing by back probing and refering to a volts vs kPa or inHG chart. Clogged passage would show bad map sensor even if you swapped. If all sensors are spec-ing out, passages are clean, replace the ecm.

As far as comp, check it cold. Checking comp hot is generally going to give false info. Once the block is hot, the comp will always be higher.

I'm leaning towards a harness fault, possibly a ECM based on the issues you have and what you have checked so far. I've replaced a few harnesses because they would do weird things. Always start with grounds on a EFI system, you eliminate the basics first or you'll waste time and money to find out you had a poor connection that takes 5 minutes to fix...
 
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Cool, I checked voltage and found a bad wire causing the inop tps. thas done and dusted. I'm going to try and comp test this weekend when I do manifold gaskets and/or brakes (actual stuff I don't mind working on).

Thin king im going to buy an aldl connector and try and run some diag on my laptop
 
Well, kind of an update:
I was able to pull data, and a couple things popped out
1. RPM the ecu is displaying is 100-200 rmp higher than actual (I've got a gauge running off of the distributor tach wire)
2. TPS % is at like 46% without me touching the throttle. idle voltage is about .9 which I know is high, but I would think the tps % would be closer to zero even at .9v
3. It will go into closed loop fueling which I thought wasn't happening from the 1 wire o2
4. Not sure if this is a standard parameter or just with TunerPro, but the Intergrator does dip below 100 (assuming this is some sort of parameter related to engine RPM)
5. Knock sensor count was about 13 after a 15 minute drive. Not sure what this is driven by as theres no audible knock. It does missfire a little bit on cold starts, but thats about it.

Takeaways are maybe my distributor is bad sense its sending two different signals to the gauge and the ecu. Not sure what's up with the tps or the knock though. I'll try a different sensor and see what happens
 
1) Bad pickup sensor in distributor possibly. Also a possible ground issue. Analog tachs are generally poor at accuracy, I've seen tachs that read 900 rpm when the actual rpm was 650 so I wouldn't take this too seriously. the gauge might just be wonky.

2) Adjust the tps volts down, disconnect battery, then recheck. The later model trucks had a little solenoid that would open the throttle lever to act as a warm up choke. if yours has that, make sure it kicks down after warm up.

3) Closed loop is just a fuel/timing map based on coolant temp, map and throttle position. the o2 sensor is telling the ECM what the oxygen in the exhaust looks like, so it adds and subtracts fuel to fine tune the cruise A/F ratio. That's how a speed density system works.

4) I dont have experience with tuner pro so no input on this.

5) The knock sensor isnt looking for rod knocks, its looking for preignition or detonation in the cylinders due to lean fuel or bad timing.

Outside of the TPS, Im starting to think your system is running lean. It would explain the low vaccum and RPM at idle but not when the throttle blades are opened. It explains the knock count and also gives a reason for the cold start misfire. .030 bore over is a big step up for the early speed density systems. A maf system could compensate better but, the MAP sensor cant adjust for more displacement without being calibrated to do so. Did the PO install a aftermerket PROM chip? That might be your issue. Imagine a carb truck with a lean idle mix...idles low or stalls, runs ok with throttle open.
 
Gm does some stupid default to a 4 cyl tach signal (they do this on the LS Too). (mine was also higher via watching on WinAL/DL than what showed on my tach)

Fix the TPS voltage that's #1. Then do an IAC reset.
 
PurfsMcGurfs said:
1) Bad pickup sensor in distributor possibly. Also a possible ground issue. Analog tachs are generally poor at accuracy, I've seen tachs that read 900 rpm when the actual rpm was 650 so I wouldn't take this too seriously. the gauge might just be wonky.

2) Adjust the tps volts down, disconnect battery, then recheck. The later model trucks had a little solenoid that would open the throttle lever to act as a warm up choke. if yours has that, make sure it kicks down after warm up.

3) Closed loop is just a fuel/timing map based on coolant temp, map and throttle position. the o2 sensor is telling the ECM what the oxygen in the exhaust looks like, so it adds and subtracts fuel to fine tune the cruise A/F ratio. That's how a speed density system works.

4) I dont have experience with tuner pro so no input on this.

5) The knock sensor isnt looking for rod knocks, its looking for preignition or detonation in the cylinders due to lean fuel or bad timing.

Outside of the TPS, Im starting to think your system is running lean. It would explain the low vaccum and RPM at idle but not when the throttle blades are opened. It explains the knock count and also gives a reason for the cold start misfire. .030 bore over is a big step up for the early speed density systems. A maf system could compensate better but, the MAP sensor cant adjust for more displacement without being calibrated to do so. Did the PO install a aftermerket PROM chip? That might be your issue. Imagine a carb truck with a lean idle mix...idles low or stalls, runs ok with throttle open.
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How do you adjust tps voltage, I thought that was based on sensor input?

I'm hoping to address the low vacuum soon. I have new manifold gaskets and if that doesn't fix it, I'll probably swap to a stock cam since I have a hunch this guy didn't just bore the motor out and may have thrown a wacky cam in there that's messing with the ecu (assuming it still has the stock chip). No clue on PROM chip either. I'll pull the ECU and check to see if its a stock chip or not.
 
vrtillman said:
How do you adjust tps voltage, I thought that was based on sensor input?

I'm hoping to address the low vacuum soon. I have new manifold gaskets and if that doesn't fix it, I'll probably swap to a stock cam since I have a hunch this guy didn't just bore the motor out and may have thrown a wacky cam in there that's messing with the ecu (assuming it still has the stock chip). No clue on PROM chip either. I'll pull the ECU and check to see if its a stock chip or not.
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You have to loosen the screws in the TPS sensor and rotate it. Do this so the voltage you see with your throttle closed at idle position is .02 V -.05 closer to zero the better.

(not running) sometimes you have to slot the holes in the TPS sensor to get it adjusted right.
 
On my '87 GMT-400 a similar issue was caused by a sloppy throttle shaft. A rebuilt TBI fixed it.
 
Well I thought I'd provide an update, still no fix. New distributor and new spark control module didn't fix the issue. Also put a new oil pump and timing chain in for good measure. I changed every gasket except the head gasket (no reason to) so it wasn't a leaking manifold gasket. The only sensors/modules that are still original or haven't been tested for functionality are knock and oil pressure so I'm thinking I might just change those for shits and giggles. And I guess I forgot to mention that the ecu/prom chip are stock (from what I can tell). I might try and find a duplicate from a wrecked truck to try and see if that's what is causing the issue.

I do have a spare throttle body, but there's no noticeable play in the current throttle shaft and it passes the carb cleaner test so I don't think that's the culprit.

I also discovered that the engine in the truck is an older style block (2 piece rear main seal, but 4 bolt main!) so maybe this guy did throw a goofy cam in there. There weren't any markings on the end of the cam from when I replaced the timing chain so who knows. from doing the pan gasket, I could tell there were aftermarket pistons (was told it was bored .030 over) so that at least checks out.

My brain also went to maybe there's a high stall torque converter in there that is causing some initial parasitic drag on the crank? Idk, I'm not an auto trans expert, just a thought. At this point, I'm about to go pull a tbi small block from the junk yard and run that while i get everything ready for an eventual 6.0/4l80e swap. At this point I just want something that works.
 

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I have learned from research and questions getting answered on here, that it is important that the knock sensor wire is in good condition and NOT touching spark plug wires.

The knock sensor torque is very low and over torquing it will destroy it, so keep that in mind.
 
If you know it is bored over, then the speed density system can not compensate for more fuel. It's worse if the cam isnt stock. Find a custom prom chip for your truck. Try Harris tuning or google tuned prom chips for GM. You'll need to know the cam specs to get it right.

Making a 454 into a 460 doesnt seem like a lot but, emissions in those days started getting strict. That's why a 454 only made like 190 hp in the 90s. They ran them lean. Try spraying starting fluid or dripping gas in the tbi and see if it runs better.

The truck gave you clues with the map code, knock counts and driveability. You verified it's not stock. Tine to fix the fuel map. Throwing parts at it isnt going to fix a programming issue. Its like running a stock lq4 ecm on a cammed ls3. It will never run right.

Edit: also, a high stall converter wont lock up at a low rpm, that's the point. It stalls higher to let the engine rev up more for the cam's powerband. Stock converters are usually 1800-2200 rpm but, I dont remember for sure.
 
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PurfsMcGurfs said:
If you know it is bored over, then the speed density system can not compensate for more fuel. It's worse if the cam isnt stock. Find a custom prom chip for your truck. Try Harris tuning or google tuned prom chips for GM. You'll need to know the cam specs to get it right.

Making a 454 into a 460 doesnt seem like a lot but, emissions in those days started getting strict. That's why a 454 only made like 190 hp in the 90s. They ran them lean. Try spraying starting fluid or dripping gas in the tbi and see if it runs better.

The truck gave you clues with the map code, knock counts and driveability. You verified it's not stock. Tine to fix the fuel map. Throwing parts at it isnt going to fix a programming issue. Its like running a stock lq4 ecm on a cammed ls3. It will never run right.

Edit: also, a high stall converter wont lock up at a low rpm, that's the point. It stalls higher to let the engine rev up more for the cam's powerband. Stock converters are usually 1800-2200 rpm but, I dont remember for sure.
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I don't think that a .030 over 5.7 is his problem. We had a 5.7 rebuild bored .030 10/10 crank in a '89 k2500 and it ran good until it lost a headgasket due to a shit head install.
 
M92PV4U said:
I have learned from research and questions getting answered on here, that it is important that the knock sensor wire is in good condition and NOT touching spark plug wires.

The knock sensor torque is very low and over torquing it will destroy it, so keep that in mind.
Click to expand...
I also don't "think" that this is your problem, but if you are going to change it check the wire and don't destroy the new knock sensor upon installation.
 
vrtillman said:
I do have a spare throttle body, but there's no noticeable play in the current throttle shaft and it passes the carb cleaner test so I don't think that's the culprit.
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Open the throttle a little bit and then check for play in the throttle shaft.
 
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