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Old 04-06-2011, 11:44 AM   #26
maynardtruckin
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Re: 89 tbi running & starting problem

The plugs are black.
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Old 04-06-2011, 12:47 PM   #27
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Re: 89 tbi running & starting problem

Which coolant temp sensor did you replace?
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Old 04-06-2011, 01:11 PM   #28
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Re: 89 tbi running & starting problem

the one on the intake manifold not the one on the side of the head that runs the gage.
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Old 04-06-2011, 04:39 PM   #29
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by maynardtruckin View Post
The plugs are black.
Does the Truck have A.I.R. (Air Injection Reaction) aka a smog pump?

Here is what I would do:
Install clean spark plugs.
Get it started without allowing it to load up.
Watch the sensor data with a scan tool to see what is going on.


If I had no access to a scan tool, I would install clean plugs and run the truck with the O2 sensor unplugged to keep the truck from going into closed loop operation, and see how it ran warm in open loop.
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Old 04-06-2011, 05:52 PM   #30
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Re: 89 tbi running & starting problem

no air
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Old 04-06-2011, 08:42 PM   #31
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Re: 89 tbi running & starting problem

it still runs the same with new plugs and the o2 unplugged. Ever sense I reset the timing I will use the starting fluid and fire it up it runs for a min and dies. When I get it re fired it runs fine after some times and others it has a miss. still falls on its face when you gas it hard from a idle.
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Old 04-06-2011, 10:10 PM   #32
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Re: 89 tbi running & starting problem

Is the gas in the tank, old gas, that came with the truck?

Did it soot the new plugs already?
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Old 04-06-2011, 10:13 PM   #33
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Re: 89 tbi running & starting problem

no new gas and yes plugs are covered
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Old 04-06-2011, 10:23 PM   #34
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Re: 89 tbi running & starting problem

dont know a lot but i have the same diagnosis. with that in mind i have had a couple of people tell me that the catalytic converter. have you checked the cat?
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Old 04-06-2011, 10:27 PM   #35
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Re: 89 tbi running & starting problem

cats fine
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Old 04-06-2011, 10:34 PM   #36
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Re: 89 tbi running & starting problem

not ruling out bad parts but they test good so who knows.
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Old 04-06-2011, 10:37 PM   #37
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Re: 89 tbi running & starting problem

The old gas could be a big part of the problem.

If the coolant senor ground is poor it will cause a cold temp reading which will cause the system to be to rich. The ground is to the front of the intake manifold. The ECM also grounds at the front of the intake.

If the MAP sensor ground is poor it will cause it to go rich. You said it made no difference when you unplugged the Map sensor. You should test it.



MAP sensor testing TBI

Manifold absolute pressure sensor

Do you have a hand vacuum pump for testing the MAP sensor?

A normal Map sensor reading on the wire that connects to terminal B (possibly Lt green) would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

NOTE: if it runs poorly it could have much lower vacuum at idle which would cause a higher voltage reading.

Terminal A (possibly Gray) is voltage supply to the Map sensor. (Should be close to 5 volts)

Terminal C (possibly purple) is the ground for the Map sensor. (Should have no voltage)
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Old 04-06-2011, 10:44 PM   #38
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Re: 89 tbi running & starting problem

will check that tomorrow.
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Old 07-29-2017, 04:56 PM   #39
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Re: 89 tbi running & starting problem

Hello Folks! I'm new here & after many of the same maladies described here in dealing with my 1991 3500 7.4L TBI to no avail. I made the switch to a 780 Holley Carb & Edelbrock alloy manifold with a brand new HEI distributor & curve kit. ALL my problems are behind me now! My gas mileage & performance improved tremendously The truck INSTANTLY starts ( it used to crank forever with the
tbi) & runs smoothly & will fry the tires if I desired. Had to install a Holley fuel pressure regulator & a variable volume flow control for the return restriction to achieve 6.5 lbs fuel pressure. Using the Holley TPS adapter kit I have NOW FULL control of the engine functions & transmission shift points By removing the GATEKEEPER (computer) the truck is predictable & has MORE towing power than ever before. I have performed this swap sever times in the past when confronted with these kinds of problems & have had NO complaints with the results. This means HAPPY CUSTOMERS!!! No comebacks. I went through replacing everything electronic on the truck WAISTING almost a grand trying to fix the electronic problems till I was sick of driving it the way it was. I know some of you want to keep what you currently have, but this modification solves all the headaches. (besides that I'm an old school Hot Rodder with years of modifying all kinds of cars & boats) I hope this will help someone with the problems I've encountered & I'll gladly provide detailed instructions for the swap if anyone so desires. As always Have a GREAT DAY!! GONMAD
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