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GMC vandura with 350 tbi dies when warms up

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GMC vandura with 350 tbi dies when warms up

Vandura Vandura
New User | Posts: 4 | Joined: 04/11
Posted: 04/04/11
10:57 AM

Good day folks.

I have purchased a box van GMC vadura year '90 ,it has a 350 tbi engine. It fires up and runs semi-alright (have a little knock when I open throttle rapidly) but when engine warms up it idles down and within 5-10 seconds it dies.
I checked all vacuum connections and all of them seems to be alright. Engine also have a new wires and distributor cap ( original owner was trying his best to fix it) fuel is spraying smooth from injectors as it should... If I open throttle when it idles down the engine still dies. When it's cold it runs okay.
The check engine light is off.

Please help me troubleshoot the problem. Confused

I also got one more question... As I mentioned it knocks briefly when I open throttle real fast and I suspect the ignition is advanced a little too far. How do I check ignition on the van? I know how to look through the top of the engine through the water pump with the strobe but in the van it's so tight I can hardly slide my hand over the engine much less look in there. What can I do?

Thank you


waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 04/04/11
03:13 PM

do you know anybody with a scan tool...?????

i would first start out with testing the power brake booster... pinch off the vacuum hose from the booster.. see if thats leaking..  it can leak enough to stall the motor...

next... you need to set the timing.. since this will take 2 people i have an easy way....  climb under the front of the van...  make sure the timing pointer is cleaned and visible...   make sure the damper is clearly marked....  using a 5/8, six point socket with an extension or a deep socket on the crank center bolt ... turn the crank in normal rotation like you are tightening the bolt to bring the timing mark to TDC>.. i just looked the timing spec on that motor is TDC...

now that is set...  take off the inner cover... yep.. you were going to have to do this anyway...  take off the cap and rotor...  look at the pickup coil and the reluctor teeth...  it will be close...  but you need to loosen the clamp bolt.. and turn the housing slightly... till the little tips all line up... this is where the ignition is going to fire at....      i can get this close enough to only need to check the timing...

there are instructions on the emissions label on just what you have to do to properly set the timing.. disconnecting the tan with black stripe wire... or jumping the A and B terminals on the aldl under the dash.. or front seat...

what i have described... will get it really really close...  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 04/04/11
03:41 PM

next.. after you set the timing..    

and you have the engine compartment open...

reach in and grab the throttle lever... engine off....   open it slightly...   wiggle it around... see if the shaft is loose in the throttle body ... i have found them seriously worn out...   and this can cause hard to diagnose idle problems... as the throttle can close more than  normal... and it can close to slightly different angles..

with a scan tool.. OBD1 gm...  you will want to look with the engine off.. key on.. the throttle position sensor voltages...

throttle closed... less than 1.0 volts..  more than 0.5...  work the throttle different speeds... letting it close each time... it needs to come back to the EXACT same voltage...    0.75....  0.75..... 0.75.......  if you get various numbers on closing the throttle...  you have a worn out throttle shaft bore... and rebushing the throttle body.. can be done...

with the engine started... look at your scan tool values...

for the TPS...  as above...

the ECT.....   warmed up... 190F to 215F.. if you can get it that warm...

the IAC...   20 to 50 IAC. when the engine is warmed to operating temp...  what the heck is an IAC...  idle air counts...  this is the big hex nut thing screwed into the side of the throttle body... its a stepper motor... drives a tapered plug in and out of the side of the throttle body to control the idle speed..   it controls idle speed... changes the amount of air thats bypassed around the throttle blades...   when the ECT sensor shows the engine as cold... the computer backs the IAC out to above 80 counts.. sometimes over 120...  this lets a lot of air in to raise the idle speed for fast idle..  as the engine warms up.. the computer starts moving the idle air control motor in...      normal expected position is between 20 and 50 counts... so the computer can control the idle speed either way...

when the throttle shaft bore wears... the throttle shaft can close to a different angle each time.. since the throttle position sensor is on the end of that shaft.. it sends a different voltage back to the ECM ... when it is higher than the lowest voltage seen this ignition cycle...   the ecm things you still have your foot on the gas pedal..  so it keeps the fuel mixture richer than idle settings...    will sometimes turn on the check engine light..

do you have a scan tool... i did a forum post on scan tools  for OBD1 corvette C4s over at a few weeks ago.. to help people find used inexpensive scan tools to work on these instead of pulling their hair out and swapping parts until their wallets are empty...  

Vandura Vandura
New User | Posts: 4 | Joined: 04/11
Posted: 04/06/11
12:43 AM


First of all thank you so much for detailed how to!

After I took the air filter and spacer ring off throttle body and fired it up I saw the fuel spraying from injectors.Then right before the engine started to die on me I squorted some carburetor cleaner in to the throats and engine came back to life which made me think it was starving. Now visually it was spraying fuel but as I understand now it wasn't spraying enough. Now according to the previous owner he replaced filters,plugs and wires. I crawled under the van and pulled the filter- I was unable to blow through it. Replaced it and it fired right up and continued to run.However now I discovered another problem. Actually it existed before but engine didn't run long enough for me to try to pinpoint it.

When I open throttle rapidly the engine detonates. I tried retarding timing by turning distributor but it didn't help any.

Unfortunately I don't have access to the scanner to check sensors.
Could it be the knock sensor?

Idle was a little rough so I pulled out IAC valve and it had some carbon build up ,I cleaned it-idle smoothed out(or I am paranoid over it).
I got the timing pretty close to where it would start however I still don't understand how you can look at the timing mark on balancer. I even though that there is another set of timing marks that would be visible from UNDER the van but I haven't noticed any. The manual for the van that I bought describes looking at the timing mark from the top.
I can't figure out for the life of me how to look at the mark in the van. I can barely fit my hand in there . It driving me nuts :-)

Thank you for taking time and explaining the troubleshooting procedure. I shall print it out and study it to the "T"


waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 04/06/11
05:58 PM

mark the front of the damper with a paint mark also....    you might have to use a mirror to see the timing pointer as you  turn the crank......

vans are a pain....   thats one of the reasons i described how to dead stick time it..

looks like that model has the plastic timing cover... the timing mark is at about 2 o clock postion...

but it might also have the steel cover... and thats is about 12 o clock....

so now you can at least visualize whats hidden behind the water pump......

this is probably what you distributer looks like without the cap and rotor.. line up the tips when the crank timing mark is lined up at front...  


this is a V6 reman distributer with a newer replacement style reluctor and pick up coil..  you can see where to turn the housing to line up the reluctor tips and the pick up coil teeth in this image.. (they are NOT aligned in either image)


waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 04/06/11
06:07 PM

you might also want to examine the coil for signs of high voltage leakage..


tuffnuff tuffnuff
Moderator | Posts: 7827 | Joined: 12/09
Posted: 04/06/11
08:45 PM

Nicethread 1

Nice pointers Wayne, handy stuff to know

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

Pontiacman2 Pontiacman2
Moderator | Posts: 8956 | Joined: 09/08
Posted: 04/07/11
01:24 PM

Very good post Waynep.  

Professional Hi-performance engine builder

Vandura Vandura
New User | Posts: 4 | Joined: 04/11
Posted: 05/02/11
12:58 PM


Thank you for taking time.
I set the timing at TDC and motor is still knocking under acceleration.
I tried everything from higher octane gas to seafoam in the throttle body to new oxy sensor.
I also bought a repair manual but it shows too many years that there's hardly anything said about detonation problem.

Does this motor have a knock sensor?

I did have to set timing at TDC,right?

I ran out of ideas on the knocking issue.

Do you have any ideas?

Thank you.


waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 05/02/11
11:06 PM

how did you set the timing....

there are instructions on the emission label on how to do it...    

with the engine running...    in normal operation..   you have to do something else to set the timing...

you have to disconnect a tan wire somewhere.. or jump the A and B terminals of the data link connector under the dash...

you do have to follow the emissions label.. its not written elsewhere usually...  including repair manuals...

you can dead stick time the motor...   by setting the crank damper to TDC marks.. and taking the distributer cap off and rotating the housing till the tips of the reluctor match line up with the tips of the pick up coil... exactly..

that will get you really close...

you do have to use care when setting the crank.. as you have to be rotating in the normal rotation of the crank.. or you will toss in any slack in the timing chain into the mix and you will NEver get it right..

with a scan tool plugged in..  you can see the detonation sensor retard...   you can tap the block with an antenna tip and see if the timing retards...

while you have the distributer cap off...   grab the shaft.. wiggle it... does it wiggle....  did you get a look at the distributer reluctor shown above.. that will make it ping... detonate...  

Vandura Vandura
New User | Posts: 4 | Joined: 04/11
Posted: 05/19/11
08:09 PM


I first disconnected the brown/black wire from the distributor. It immidiately threw a code . I then removed the windshield washer fluid reservoir and managed to look at balancer with use of small size mirror. I used a strobe on #1 cylinder and turned the distributor to the 0 degrees tdc as mentioned on the emission label. Then I killed motor,reconnected the brown black wire and fired it back up.
The knocking problem became less noticable although still very evident especially under load in certain position of throttle... For example I would start out without any problem, drive down the road (im guessing go out of the closed loop) and motor would start detonating. If I lay off the gas it would improve the detonation problem ,at the same time if I floor the gas pedal it will somewhat improve the knocking as well. The oxygen sensor is brand new,so are the sparks and wires and cap. The motor was treated with sea foam before  ireplaced the o2 sensor.

I also checked out base gasket for the leakage by spraying first WD 40 then butane from the torch. No changes in the motor speed or sound when spraying.
I checked the shaft for play-it feels nice and tight,no play in there either. The temperature sensors are both connected.I double checked ,cleaned the terminals of sensors...

Anymore ideas on what could cause the detonating?

Is there a knock sensor on this engine? I haven't found it and book does not mention about it .

Thank you for taking time and helping me out.  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 05/19/11
08:58 PM

were you able to remove the distributer cap... pull the rotor...   examine the reluctor magnet????  see if there are any cracks...

shown above.. the rusty picture above...
this can cause detonation as its throws out of phase timing pulses..

dead stick timing verification...  set damper mark to TDC... does not matter which rotation its on... tips of the reluctor must line up perfectly with pick up coil...  this is just a verification ...   since you will have the cap off again..

question.. are the last two cylinders on the drivers side spark plug wires separated??? so they are at least 1 1/2 inches apart...  so they don't induce a spark across...  it has been known to happen before..

yes... these should have a detonation sensor...  screwed into the bottom of the block ...  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 05/19/11
09:10 PM

knock sensor...

the proper shaped connector for the detonation sensor...


spark control module..

there is a manual section for checking the detonation sensor operation..  almost any gm manual will have Chart C-5 on how to diagnose and test the spark control system...  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 05/19/11
09:20 PM


Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system.

During cranking, the timing is controlled by the ignition module while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.

An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.

The conditions for setting this code are:

System in BYPASS mode but the ignition module is still controlling timing
  - or -
Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.

Typical causes for this code include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM



Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit.

ESC is used to sense spark knock (pinging) and retard the timing to eliminate it. A knock sensor (located at the rear of the engine block) sends signals to an ESC module which then signals the ECM that knocking is being detected. The ECM will retard the timing by as much as 20 degrees in 1 degree increments. A loss of knock sensor signal or loss of ground at the ESC module will cause the signal at the ECM to remain high. The ECM will act as if no knock is present, and may possibly result in engine damage, if there is detonation.

Loss of the ESC signal to the ECM will cause the ECM to constantly retard the timing to its maximum. This results in sluggish performance and a Code 43.
The conditions for setting this code are:

Engine is running
ESC input signal has been low more than 2.23 seconds

Typical causes for this code include:

1) Open or shorted knock sensor
2) Loose knock sensor
3) Excessive mechanical noise within engine
4) Improper or incorrectly installed PROM or CALPACK in the ECM or defective ECM
5) Intermittent open in the EST line to the ignition module  

waynep7122 waynep7122
Guru | Posts: 1130 | Joined: 08/09
Posted: 05/19/11
09:30 PM




See Figure 1

Located in the engine block, the knock sensor retards ignition timing during a spark knock condition to allow the ECM to maintain maximum timing advance under most conditions.

Fig. Fig. 1: Exploded view of the knock sensor location - 4.3L, 5.0L, 5.7L and 7.4L engines


Connect a timing light to the vehicle and start the engine.
Check that the timing is correct before testing knock sensor operation.
If timing is correct, tap on the front of the engine block with a metal object while observing the timing to see if the timing retards.
If the timing does not retard the knock sensor may be defective.


Disconnect the negative battery cable.
Disengage the wiring harness connector from the knock sensor.
Remove the knock sensor from the engine block.

To install:
Apply a water base caulk to the knock sensor threads and install the sensor in the engine block.

Do not use silicon tape to coat the knock sensor threads as this will insulate the sensor from the engine block.

Engage the wiring harness connector.
Connect the negative battery cable.  

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