RTH diagnosing 1990 350tbi stumble/stall when warm (1 Viewer)

jfz80

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Ok so truck is two hours from me and I spent the last two days working on it to no avail. I am home briefly to do some searching/regrouping and will be heading back asap to tackle it this week. sorry for the long post but trying to be thorough and brief as possible.

background:
1990 gmc TBI 5.7. SM420 Daily driver 10k miles a year. Leaks
I purchased the truck in 09. The 350 has been in place for over 10 years and has driven fine while I have owned it minus an alt. snafu.
THERE IS NO CEL-- as in not installed, anywhere, not just unlit
This makes codes hard to come by unless I get the ALDL cable interface for a laptop or someone knows which circuit I could tie a light bulb into to read. The OBD1 readers just flash your CEL to get the codes just like jumpering the ALDL but again there is not CEL lamp.

Issue:
Drove fine all last week. Filled up friday morning and noticed a fuel smell while driving b/w meetings around town. Checked lines/unions/filter/pump for leaks and there were none. 5pm friday traffic and it began to stumble and idle dropped and stalled at a red light. Pushed to safety and could not start. It turns over fine, fires, then promptly dies. Fuel pump was making a whining noise that was new at this time. A day later I was able to get it started and keep it running by feathering the gas and keeping revs up. bucked horribly under load in 1 and 2 but once up to hwy speed did pretty good with exception of lack of power(stumble under load) max speed of 60 and fuel smell. Longer it ran better it drove but didnt have the confidence for 2 hrs of hwy.

Actions so far:
Not a fan of shotgun approach mostly for the price of the ammo but stuck out of town limits your options. So far I have done the following.

New fuel pump-- electrical 25-30 psi-- replacement for part that had been installed and working since purchase
*edit* seems the clerk was looking at the wrong specs..... google shows 5-9 psi for this unit**
New Fuel filter
New injectors in TB (old were corroded and had odd spray pattern)
Replaced all gaskets/seals/diaphragms in TBI unit as well
Replaced all vacuum lines and even the plugged port caps too.
New intake manifold gaskets (mostly to fix the leak) Tstat/dizzy too
New coil
New Distributor/ecm/cap rotor (old tested fine but was horribly corroded)
set base timing to zero with set timing plug disconnected, reset ecu


Tested TPS-- in spec .534 closed and 1700 open
Tested IAC-- jumpered ALDL and can hear and feel IAC plunger move in the tb-- could not verify its travel but could feel it open close
Tested EGR by applying vacuum and diaphram moves freely
Tested MAP sensor with vacuum and it reponds to different levels


The truck starts and runs great untill it warms up. Then the idle begins to slow and eventually it stalls. I advanced the timing to 4 then 6* and it would run better with greater throttle response (less delay) however it was not driveable. It runs fine but when under slightest load (like disengaging clutch 1/2/3rd gear) it bucks/surges horribly. Strangely and maybe unrelated it drove quite well in reverse.

My thoughts:

really Im out of good ones, hence the borrowed car and long post
IAC has not been replaced and I have read multiple times a failed IAC will cause the fuel pump to go out. However my pump just sounded horrible yet seemed to provide plenty of fuel, replaced it for the previously broken and rigged + terminal. I can hear IAC activating in the tb and stuck a pick down there to make sure the plunger was actually moving and it was. Still a suspect in my mind

Cooant temp. sensor? maybe affecting the ECU with wrong temp data? Especially since it occurs after warm up or enriching cycle.

Any and all help very much appreciated guys
 
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Pull the ground and walk away for a few min. Jump the ALDL and make no codes are present. Remove jumpers. Run the motor till you get the described issues. Check ALDL.

GM TBI only needs 19-30psi to really work properly. You can get away with higher, but you might just flood your injectors. Have your NEW coil tested to see if its working. Then test your power connectors to see if the coil is getting power. I would think the temp sensor could be calibrated with the O2 sensor, but would have to be adjusted via TPS.
 

Trollhole

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Check engine light (service engine soon) should be the brown/white wire coming out of the ecu. Connect one side of light to wire and the other side to 12v +. Then run the codes and report back.

When you turn the ignition to on do you hear the fuel pump engage for a couple seconds?

Get yourself a fuel pressure gauge and report back the flow.

You can also put a timing light on the main coil wire and use it to check the spray pattern.

Also can connect it to any plug wire and fasten the timing light on the hood where you can see it and go for a ride. Make sure the light isn't skipping around that would tell you if it's spark.


My guess is it's spark related. That or you fuel pressure is crazy high or crazy low. Take some compressed air and blow through the lines on the tank side of the fuel filter and pump and make sure there is not a clog in the tank.
747ecm1.jpg
747ecm4.jpg
 
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inkpot

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Howdy! My son used to have a 90 Camero with a 350-tbi. It had some similar issues. First thing off the top of my head was that it had an aftermarket air cleaner that used a plastic neck/spacer to raise it up above the throttle linkage. After much grief, found that when it was warmed up, the plastic spacer got just soft enough that the back edge of the air cleaner body rested on one of the terminals of the coil. That would create a random/intermittent short, and it would run like crap, part time. Took us forever to find it. Had some other issues too, but I can't remmeber the cause. I'll keep banging it around in my head for a while. John
 

jfz80

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Pull the ground and walk away for a few min. Jump the ALDL and make no codes are present. Remove jumpers. Run the motor till you get the described issues. Check ALDL.

GM TBI only needs 19-30psi to really work properly. You can get away with higher, but you might just flood your injectors. Have your NEW coil tested to see if its working. Then test your power connectors to see if the coil is getting power. I would think the temp sensor could be calibrated with the O2 sensor, but would have to be adjusted via TPS.
Thanks for the info. No light to be able to check codes but THC has helped me find the ECU pin to solve that in his response. The new coil was tested before leaving the store. I only mention CTS because it keeps showing up on knightrider.com i mean thirdgen.org. for those with similar symptoms
from a gm service tech "Sounds like a classic case of a bad Engine Coolant Temp sensor (located in the intake manifold near the tstat). It tells the ECM the temp of the engine and the ECM uses this information to control the amount of fuel delivered to the engine. Cold needs more fuel than warm.
In closed loop, Throttle Position, MAP Sensor reading, CTS reading and O2 are used for fuel calculation.
In open loop, the CTS is also used for fuel calculation but the 02 is ignored in open loop
This explains the onset of issues when engine reaches operating temps.



Check engine light (service engine soon) should be the brown/white wire coming out of the ecu. Connect one side of light to wire and the other side to 12v +. Then run the codes and report back.

When you turn the ignition to on do you hear the fuel pump engage for a couple seconds?

Get yourself a fuel pressure gauge and report back the flow.

You can also put a timing light on the main coil wire and use it to check the spray pattern.

Also can connect it to any plug wire and fasten the timing light on the hood where you can see it and go for a ride. Make sure the light isn't skipping around that would tell you if it's spark.


My guess is it's spark related. That or you fuel pressure is crazy high or crazy low. Take some compressed air and blow through the lines on the tank side of the fuel filter and pump and make sure there is not a clog in the tank.
Thank you for the continued support Marshall :cheers:
+Dang now that I see that chart I realize its in the pile of pages printed yesterday.
+I do have continuous fuel pressure when key is on and have a fuel pressure guage as well to return with.
?? I can check delivery pressure from pump to tbi in line. how do I check pressure after regulator in TBI?
+Spray pattern is MUCH improved with new injectors. Driver side still seemed to have a bit of a dribble within the spray but a nice even cone.
+Great advice. Have light and duck tape, will give it a shot.
+Thanks again for the help, I can drain tank and check lines, though I will say it flowed freely via gravity when changing the fuel pump.


Howdy! My son used to have a 90 Camero with a 350-tbi. It had some similar issues. First thing off the top of my head was that it had an aftermarket air cleaner that used a plastic neck/spacer to raise it up above the throttle linkage. After much grief, found that when it was warmed up, the plastic spacer got just soft enough that the back edge of the air cleaner body rested on one of the terminals of the coil. That would create a random/intermittent short, and it would run like crap, part time. Took us forever to find it. Had some other issues too, but I can't remmeber the cause. I'll keep banging it around in my head for a while. John
Howdy John. thanks for chiming in with your experience.
I have feared this would be a 500 dollar repair for a 5 dollar part.

I have the exact setup you describe. However my issue remains that it will stall while idling when the engine warms up even without the air cleaner on the vehicle. Definately still got me thinking about it as a culprit for it ran much better when advanced to 6* but was not driveable for the surging/bucking under load which may be the cleaner housing torquing to make contact with the coil if it wasnt excessive timing.
 
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jfz80

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Gap was correct and wear was even, tips still shaped well. They were dry after a couple hours of idling and testing. Orangy/tan

I buried a question in my above response that Ill repeat here.

How do I measure fuel pressure after the regulator?
GM needs 10-15 psi of supply and 9-13 (10.5 ideal) fuel pressure to injectors.
A fuel pressure guage T'd into the supply line b/w pump and tbi would report pump/supply pressure. Is there another device or method to measure PSI post regulator or am I misunderstanding the distinction b/w the two.

In rebuilding the regulator I did notice a 1/8'' washer/spacer that I have read today should increase PSI by 1.5 over stock which is known to be on the low side.
 

Trollhole

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Gap was correct and wear was even, tips still shaped well. They were dry after a couple hours of idling and testing. Orangy/tan

I buried a question in my above response that Ill repeat here.

How do I measure fuel pressure after the regulator?
GM needs 10-15 psi of supply and 9-13 (10.5 ideal) fuel pressure to injectors.
A fuel pressure guage T'd into the supply line b/w pump and tbi would report pump/supply pressure. Is there another device or method to measure PSI post regulator or am I misunderstanding the distinction b/w the two.

In rebuilding the regulator I did notice a 1/8'' washer/spacer that I have read today should increase PSI by 1.5 over stock which is known to be on the low side.
You don't cannot unless you want to put the gauge on the return line. It should be almost zero. If it's high then you have a restriction in the return line.
 

jfz80

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I guess im confused on how you determine if your Fuel Pressure Regulator is functioning then.

if you tap into the supply line before the TBI it seems your reading pump pressure.
Is there a way to determine if I am in the correct range of 9-13psi after the regulator?
I did rebuild it (twice) with a new diaphragm. It still has the factory non adjustable regulator, but I have read several times the modification to make it adjustable.
 

jfz80

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And we might be getting somewhere....

Carquest sold me there FP e8012S pump stating 25-30 psi however I have googled it and only come up with 5-9 variable PSI from this unit. Im thinking this is a better Carb unit than TBI.
 
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And we might be getting somewhere....

Carquest sold me there FP e8012S pump stating 25-30 psi however I have googled it and only come up with 5-9 variable PSI from this unit. Im thinking this is a better Carb unit than TBI.
But wasn't it working just fine with this unit up until just recently?
 
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I took the time to see if I could recreate your problem with one of our work vans. I pulled the temp sensor wire and made a switchable splice between a junker spare sensor. Had to use alligator clips for ground. I got motor to running temp and then shut it down, used IR to measure heat on ehaust manifold. Then flipped toggle to feed from spare that was being warmed by heat gun. IR read temp on sensor as 205degF, it didnt stumble or act funny, just let motor run lean. Verified by lean by measuring temps off manifold with IR and reading temp on aux temp gauge on panel. I couldnt take it for drive so I honestly cant recreate everything fully. I just wanted to see if I could pop a code or make it stumble at idle to 1900rpm.

Kinda stumped now man. Sorry I couldnt be more help.
 

jfz80

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But wasn't it working just fine with this unit up until just recently?
No. Sorry if that was unclear. I broke down friday pm and first course of action was a new fuel filter and this fuel pump. The old pump was unidentifiable to match but clerk thought it looked like a chevy diesel replacement pump.

I took the time to see if I could recreate your problem with one of our work vans. I pulled the temp sensor wire and made a switchable splice between a junker spare sensor. Had to use alligator clips for ground. I got motor to running temp and then shut it down, used IR to measure heat on ehaust manifold. Then flipped toggle to feed from spare that was being warmed by heat gun. IR read temp on sensor as 205degF, it didnt stumble or act funny, just let motor run lean. Verified by lean by measuring temps off manifold with IR and reading temp on aux temp gauge on panel. I couldnt take it for drive so I honestly cant recreate everything fully. I just wanted to see if I could pop a code or make it stumble at idle to 1900rpm.

Kinda stumped now man. Sorry I couldnt be more help.
Thank you for the effort to help!!! those are the great kind of tricks I dont have the engine experience yet to come up with. Thanks again.
 
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You did mention it... I just don't read so well ;)

Well, if the pump went TU to the point where it couldn't push enough fuel, and then replaced with a pump that couldn't deliver enough fuel, the original problem may still exist.
 

Trollhole

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And we might be getting somewhere....

Carquest sold me there FP e8012S pump stating 25-30 psi however I have googled it and only come up with 5-9 variable PSI from this unit. Im thinking this is a better Carb unit than TBI.


If it looked like this and you paid less than 80 dollars for the pump. Well, that dog won't hunt. And this would be the whole issue.

Airtex 110-E8012S - Airtex External Electric Fuel Pumps - Overview - SummitRacing.com



That might be your problem.
 
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I have this one that is really nice and rebuild-able that you could try if you want. I just removed it from my truck yesterday to install a high-pressure pump for my TPI. It states a maximum pressure of 12 psi, and I used a regulator to bring it down for a carb. You could run it without the regulator and you should get 12 psi before the TBI regulator. I can meet you just outside of Cola if you want or ship it. $50 and it's yours.

Mallory Ignition 4140 - Mallory Comp Pump Fuel Pumps - Overview - SummitRacing.com
 

jfz80

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GRRRRRRRRRRRRRR :bang:


yeah. thats the one. Gary sure was nice and helpful but I suppose he looked at too many pumps and got some specs mixed up. Plus in his defense I couldnt tell him the model I wanted just the psi range.

Will be heading back tonight to tackle it in the morning. Will be bringing new pump, fuel pressure guage, and likely an IAC and CTS just in case so I dont have to make 5 trips to the parts store.

Thanks again fellas, will keep you posted.
Belmont... quick ? though im not sure I could get that pump from you in time unless I drive to meet you but gas may kill that idea.
This is whats listed for your pump. So you just make sure the bypass is open to achieve the 12 psi instead of 6? Or does it push 12 all the time an the regulator brings it to 6? thanks
# Specifications:High pressure, 12-volt fuel pump
# Delivers 140 GPH free flow, 140 GPH at 6 PSI
# Internal Bypass Regulator set at 12 PSI
# 3/8'' NPT Inlet/Outlet, 3/8'' or larger fuel line recommended
# Includes fuel pressure regulator 650-4207.
 
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I am not 100%, but a quick call to Mallory would probably clear it up. I believe the regulator brings it down and the pump puts out 12 psi.

Where are you and where is the truck?


Jeremy
 

jfz80

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Cool. Working on it now.

* mallory confirms it pushes a constant 12 w/out regulator. Suggested the 140 model which pushes 15 but said 4140 will work, just that the higher the better within range usually helps TBI units**

Im in Greensboro NC and truck is in Raleigh. Both about 3 hours from you. I may have you ship it to Raleigh and go down on Friday instead but tomorrow works so much better.
 
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I posted a graph on the UC thread thinking I was on this thread. It looks like a 14 psi max, but a 12 psi max operating range. Looks like 100 gph at 12 psi is the setting from the factory and the regulator brings it down.

I can mail it to you and if it works either mail me payment or mail it back with the postage it cost to send it. Or you can get it to me some other way as I am not in a rush to get it back if it doesn't work out.

Jeremy
 

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