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GTP wont start.


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1991 Pontiac Grand Prix GTP 3.4l LQ1

Middle of a coolant change, flushing through the distilled and the engine just dies. No sputter, no pops or farts, quits like the key was turned off.  Haven’t found the issue yet.  Running out of things to trouble shoot.

things I know:

- tank has  fuel

- bought a fuel pressure tester (always wanted one) fuel pressure is tested at 46psi with key on -pressure drops to 35ish within 5-10 minutes and 20 in an hour. I don’t think a pressure drop is normal or if it is not sure how long it should hold pressure for.

- front 3 injector harnesses tested for continuity and cycling (I have an injection system tester).

- ignition system producing spark. Verified with a spark tester.

- it fired briefly after I dumped fuel into the cylinders using the injector tester.

- sprayed some quick start into the throttle body and It ran for 2-3 seconds so more proof the ignition system is good.

- I have an injector cleaner setup. Tried it out at 45psi to bypass the cars system and the car let out all the gas from the injector cleaner tank.  Seems that may indicate a bad check valve at the pump but this only occurred after I cycled the key not right away.

 if the FPR is bad I can’t imagine the engine would just up and die like that so not completely convinced.

anything else im missing I should check? Seems like this should be a simple issue but here we are. It’s acting like it’s not getting any fuel or spark but my tests say it’s good so far.

It was full of distilled water when it was running and died. Also had a check engine light on after I started it to flush the distilled water through, found out I had not reconnected the IAT after removing the intake had a code 23. Not that this would cause it to quit. Makes me wonder if it made the ecm mad and it fried in protest.  I’ve checked all harnesses again and double checked the MAP sensor harness.

WTF. Really looking forward to that ‘duh how did I miss that’ moment.

Edited by GTP091
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Posted (edited)

It isn’t the ECM no change with a replacement.  Will do the ECM Testing procedure anyway as it leads to covering the ICM as well.  What a pain in the ass the clips are to release on the ECM harness.  

im starting to think the FPR is the issue or at least I hope it is.  rather remove the plenum than take down the tank.

Edited by GTP091
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Possible the oil pressure switch or it’s connection are the issue.  The diagnostic for code 54 makes me want to test the  oil pressure circuit to see if the pump maintains pressure while cranking as well as the relay circuit.  More reasons to get more beer.

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Being it's a '91 and tried to fire when you added fuel: Ohm test the injectors. Rochester injectors back then were known for shorting out.

 

Does your fuel pressure tester have a bypass valve on it so that you can test fuel volume by the way?

Edited by mfewtrail
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Posted (edited)
5 hours ago, mfewtrail said:

Being it's a '91 and tried to fire when you added fuel: Ohm test the injectors. Rochester injectors back then were known for shorting out.

 

Does your fuel pressure tester have a bypass valve on it so that you can test fuel volume by the way?

Ohm testing the injectors is next on the docket for tonight. Or at least the front  bank anyway.

i do have a bypass valve on the fuel pressure tester.  Looking for a hard stream I’m taking it? I’ll do that when I pinch the return line and check the pressure.

Edited by GTP091
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I’ve ruled out the fuel pump circuit.  The GM testing procedure for a code 54 is good for that, even though I don’t have a code 54.  All circuits checked out.

next up testing the ecm and icm. But first the fuel pressure when blocking the return line.

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Fuel pressure when blocking the return line builds and holds at 60psi when I stop the pump.  Manual says if it holds the FPR is bad.  The pressure immediately goes back down to 46 when I release.  I’ll dick around with some ECM tests but seems this is the issue. 
 

the fuel flows nicely when I press the release valve on the tester with pump running.

Edited by GTP091
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Ordered the FPR, seals, and injector seals. Already have the Plenum gaskets.

will keep tinkering around with other things while I wait. Glad it’s not my daily driver

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Posted (edited)

Few more updates on this. Retested the front 3 injectors to be sure (back two are hidden under the plenum).  They all check out at 12.7-12.8 ohms.  All three injector harnesses test for continuity and show they’re firing using the injector tester.  Good to known if the fuel pump circuit doesn’t send 12volts back to the ECM the ECM shuts down the injector circuit. The pump, pump circuit, and oil switch circuit all test good.

I started to suspect the pump check valve might be the issue but the pressure doesn’t bleed down fast like everything I found with that issue on the interwebs.  On a hunch I hooked back up the injector cleaner kit I have and found the system will not hold pressure through the return line. If I pinch off the return line the pressure builds up to 60 psi.  If I set the injector cleaner regulator at 46 psi it leaks past the FPR.  The FPR should the flow but it doesn’t.  The return line comes out of the FPR so that must be the issue.   I had the tank cap off and it was a direct run through like bad Chinese food.  Definitely a bad FPR. 
 

parts are in now and I found an intake to plenum o ring at my local lordco so have everything i need.  So glad my car is so clean the videos I watch make me want to barf thinking about working on them.

 

glad I have the GM service manuals for this, they’re very thorough.  Except the error of the cylinder order I found in the 3.4  diagrams. Shows the banks reversed. Dumbasses… page 6D4-7 wrong!

Edited by GTP091
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1 hour ago, GTP091 said:

Few more updates on this. Retested the front 3 injectors to be sure (back two are hidden under the plenum).  They all check out at 12.7-12.8 ohms.  All three injector harnesses test for continuity and show they’re firing using the injector tester.  Good to known if the fuel pump circuit doesn’t send 12volts back to the ECM the ECM shuts down the injector circuit. The pump, pump circuit, and oil switch circuit all test good.

I started to suspect the pump check valve might be the issue but the pressure doesn’t bleed down fast like everything I found with that issue on the interwebs.  On a hunch I hooked back up the injector cleaner kit I have and found the system will not hold pressure through the return line. If I pinch off the return line the pressure builds up to 60 psi.  If I set the injector cleaner regulator at 46 psi it leaks past the FPR.  The FPR should the flow but it doesn’t.  The return line comes out of the FPR so that must be the issue.   I had the tank cap off and it was a direct run through like bad Chinese food.  Definitely a bad FPR. 
 

parts are in now and I found an intake to plenum o ring at my local lordco so have everything i need.  So glad my car is so clean the videos I watch make me want to barf thinking about working on them.

 

glad I have the GM service manuals for this, they’re very thorough.  Except the error of the cylinder order I found in the 3.4  diagrams. Shows the banks reversed. Dumbasses… page 6D4-7 wrong!

LOL, yes, I also questioned the firing order as well.  55trucker set me straight!

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Posted (edited)

 

Plenum removal procedure 1991 -1993 3.4l LQ1 :

-Drain coolant
-Open a beer 
-Disconnect Throttle and cruise cables from throttle cams
-Remove cables from bracket - lift tab to release.
-Disconnect IAT sensor
-Disconnect crankcase vent tube from intake
-Remove intake filter housing
-Disconnect TPS
-Disconnect IAC valve sensor
-Disconnect PCV tube
-Disconnect vacuum line under PCV
-disconnect EGR harness
-Remove rear spark plugs
-Remove 2- 10mm bolts from MAP sensor - pull MAP clear from plenum
-Remove 10mm bolt from fuel line clamp .
-Remove 10mm bolt holding heater core hose to plenum
-Remove clamp from short coolant line running from under plenum to crossover tube in front of engine
-Open another beer.
-Remove two 10mm bolts holding down EGR - use a magnet pickup to catch the washers.
-Jiggle EGR free and remove
-Remove two rear 13mm nuts from back of plenum- use magnet pickup to catch them.
-Remove 10mm plenum bolts 4 long + 3 short - keep them in order using cardboard piece
-Jiggle plenum to break gasket seal.
-Lift plenum up on passenger side - stuff rags between it and intake to protect mating surfaces
-Release coolant line from plenum under throttle body
-Place plenum forward to gain access to brake booster vac line - use shop cloths to support and protect it.
-Remove brake booster hose clamp at the plenum and remove the hose
-Carefully remove plenum watching EGR tube clears
-Block the intake tubes with shop cloths
-Have a beer to celebrate and inspect
-Clean everything
-Have a beer
-Fix whatever it is you needed to.
-Clean it again
 
Reinstall is the opposite of removal LOL
 
 
 
 

 

Edited by GTP091
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Found another guaranteed culprit on this no start situation. Injector #1 measured out at 1.6 ohms where all the others measure at 12.7.  I hooked up the injector to my injector tester and hit the multi fire test button and it’s completely dead.  All the others fire just fine but not going to chance it so I’ve ordered 6 new injectors.  Will be interesting to see how it runs after I replace both bad parts. 

Ill have to remove the Fuel rail but the FPR is a really tight fit so will make that a breeze now that the rail is coming off.

more beer for me! And I’ll have a chance to paint some more parts that came off while I wait.

Ill be able to add a fuel injector swap procedure to this thread now… 

Edited by GTP091
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9 hours ago, GTP091 said:

Found another guaranteed culprit on this no start situation. Injector #1 measured out at 1.6 ohms where all the others measure at 12.7.  I hooked up the injector to my injector tester and hit the multi fire test button and it’s completely dead.  All the others fire just fine but not going to chance it so I’ve ordered 6 new injectors.  Will be interesting to see how it runs after I replace both bad parts. 

Ill have to remove the Fuel rail but the FPR is a really tight fit so will make that a breeze now that the rail is coming off.

more beer for me! And I’ll have a chance to paint some more parts that came off while I wait.

Ill be able to add a fuel injector swap procedure to this thread now… 

I figured you had a shorted one from your symptoms. Even though you found a regulator that's bypassing a bit more fuel than it should, it's not enough to cause a no start imo. GM's fuel pressure spec is no more than a 5psi drop in 10 minutes time for these cars by the way. Replacing all the injectors is definitely a good idea with those.

 

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Ya it didn’t make a lot of sense to me either on why it died like that.  It has fuel pressure that held up enough during cranking so seemed like it should be enough to run:   I’m glad it is an injector as I was preparing myself to put the FPR in button it all up and have it not start.

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