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Old 01-12-2009, 07:14 AM   #1
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New user. TBI problems

Great site, I will be posting pics soon. I have picked up a 91 blue bird and in the process of converting. My question is concerning the engine. It has a 366 gas engine with TBI injection. The engine cranks up good but after a few minutes will shut off. It doesnít seem to be choking itself out due to lack of fuel, it has a new fuel pump and filter and has good pressure. I can keep it running by playing with the gas peddle. Iím concerned the return gas line is not flowing like it should. Does anybody know how much pressure is suppose to be returning to the tank? Has anybody had this problem? It may be one of them sensors also.
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Old 01-12-2009, 11:13 AM   #2
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Re: New user. TBI problems

try and see if it will run above idle with out dying , but dies when you try to let it idle, it may have a bad idle switch, just my two cents.
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Old 01-12-2009, 11:37 AM   #3
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Re: New user. TBI problems

If I hold the gas down half way it will still die out if i dont feather the pedal. same thing at idle.
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Old 01-12-2009, 08:51 PM   #4
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Re: New user. TBI problems

I love GM TBI. Hopefully I can be of some assistance. For fuel pressure you really need to be seeing 9-13 psi. This is on the feed side from the tank, NOT the return side. The return side will have no pressure on it in theory. The best place to tap in to check the fuel pressure usually is at the fuel filter since TBI trucks do not have a schrader valve. The only problem with this is that by removing the fuel filter to remove that as a possible source of restriction. That shouldn't be your problem, however.

I think you have an IAC problem most likely, but there are a few other things to check first. Can you do my a favor and measure across the bore of one of the "venturis" so I know which throttle body you have when you have a chance? Also, do you have a heated adapter under the throttle body sitting on top of an old Quadrajet style intake? Finally, how many wires go to your oxygen sensor? There is plenty of other stuff that can be done without knowing that though.

First things first...we need to establish that your TPS is reading correctly. This will work key on, engine off. With the throttle closed back probe the dark blue (I believe this should be the correct color for your engine) wire on the throttle position sensor. Backprobe it with a paper clip and grab your meter. It should read somewhere between .45 and .55 volts. Just make sure it is relatively close. Anything over say .7 volts is cause for concern. There should be a few slotted holes allowing you to pivot it to bring it back into the correct position if you need to. Gently and slowly open the throttle and make sure that the voltage increases smoothly.

The next sensor to check would be the CTS or coolant temp sensor. This will be found on the driver's side cylinder head and will have a yellow wire on it. With the engine cold you should be able to backprobe the sensor and get a pretty high voltage. The operating range is .5-4.5 volts with 4.5 volts indicating -40 degrees so I would imagine you would see 3.5 or higher depending on how cold it is where you live. If it is significantly lower or nearly 5 volts you have a problem here.

Next, check your MAP sensor. With the engine off you should have a voltage of around 4.0 volts on the light green (again...I think that should be correct) wire. Once the engine starts you should see less than that. If you have a vacuum pump you can tie put it on the vacuum line to the sensor. At about 15-20 inches of mercury you should see a voltage of 1.5vdc or do.

Finally, pull your IAC out of the throttle body. It is the large hex shaped plug at a 45 degree angle back on the passenger side of the throttle body. Make sure it is clean as well as the ports. Hopefully your bus has an ALDL somewhere on it. On that vintage truck it would be a 12 pin connector hopefully stashed under the dash. Jump a paperclip between the two upper right pins on it. That sends the computer into a diagnostic mode and part of that will be that the IAC motor will fully close. Having someone put their hand on it to feel it operate will verify this. After it has been run completely in you will need to disconnect the wiring from it. Pull your paperclip out and hope like hell that the engine will idle once it starts. With it idling you can set the minimum airflow by turning the screw under the throttle stop. You might need to punch out a small plug to do this. The emissions sticker should list the minimum idle so follow the instructions. Once that is done you need to double check the TPS and make sure it is in spec. Finally, shut the engine down and plug the IAC back in. You should have controlled idle now.

Those few things are all that are necessary to get a TBI truck started. Once it is running it is a whole different ball game and we can address that when it gets to that point. But I do want to make once final suggestion. Don't overlook the obvious stuff. Make sure you are getting a healthy spark via good plugs, wires, cap, rotor, and timing. Make sure you have good fuel. If the bus has been sitting for a long time you might just have some stale gas. Make sure fuel pressure and the filter are good. Make sure your compression is up to snuff. GM TBI really is a very simple system.

Best of luck to you.
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Old 01-13-2009, 08:41 AM   #5
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Re: New user. TBI problems

thanks for the excellent tbi tutorial
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Old 01-18-2009, 11:34 AM   #6
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Re: New user. TBI problems

Here's an off the wall one: did the bus sit parked outside for any length of time? If so, mice may have gotten in & set up housekeeping in the muffler & plugged it up.
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Old 01-19-2009, 12:55 PM   #7
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Re: New user. TBI problems

Thanks for the great replys, I havenít had the opportunity to check the stuff you asked for but will in the coming week. Iím working 2 full time jobs so itís been tough but I will get the measurements as soon as I can. As for the mice theory, I drove it about 40 miles after I bought it back to my house and it ran fine. Only when I stopped I had to pat the gas to keep it running. I'll give an update this week.
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Old 01-19-2009, 01:45 PM   #8
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Re: New user. TBI problems

this is a great trouble shooting website for tbi

http://www.chevythunder.com/gm_throt...ction_pg_1.htm
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Old 01-28-2009, 02:30 PM   #9
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Re: New user. TBI problems

ok i got 2 inches across when I messured the "venturis" since i didnt know what that was I just messured the TBI port. Hope thats it. As for the heated damper. There wasnt anything between the TBI and the manifold so i say no to that one. Im not sure which plug is the IAC. the one at the passenger side was not a hex shape but round. it was 45 degrees.I pulled it out and cleaned. I hope to get to check with the meter this week.
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Old 01-28-2009, 09:40 PM   #10
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Re: New user. TBI problems

You measured the right part. 2 inches is exactly what I needed to know to know you have the standard big block throttle body. I also now know you have the TBI-specific intake instead of the older quadrajet intake with the heated adapter. Damn the luck anyway.

The IAC will have a group of wires going to it, but the IAC itself is a stepper motor. It will be a large metal object. Perhaps a picture would help. This is a smaller bore TBI, but is roughly the same as what you're going to be looking at. No datalink connector, eh?



Have you made any other headway with it?
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