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Old 04-03-2011, 12:50 AM   #1
maynardtruckin
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89 tbi running & starting problem

Ok I have a 89 chevy 1/2 ton 2 wheel drive tbi 5.7l with manual trans. Here is the problem it will only start if I use either once it is running it is ruff idling at first the smooths out but falls on its ass when you give it gas until it dies or you feather the throttle and then rpm's pick up. Know I have replaced the computer twice, the coolant temp sensor 3 times, the ignition module twice, the fuel pump (and it has 11psi now), the throttle body once, and the engine wiring harness. It is not throwing any codes or this would be easy. Almost forgot it smells like it is running rich. So if there are any thoughts or questions let me know? Thanks!
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Old 04-03-2011, 02:33 AM   #2
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Re: 89 tbi running & starting problem

Possible that the pressure regulator has failed and it is over fueling.
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Old 04-03-2011, 09:23 AM   #3
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Re: 89 tbi running & starting problem

A 98 Vortec engine will not run on 11 psi. It has to have about 60 to 64 psi iirc. You may well have a pressure reg problem.
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Old 04-03-2011, 09:46 AM   #4
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Re: 89 tbi running & starting problem

Quote:
89 tbi running & starting problem
Quote:
Ok I have a 98 chevy 1/2 ton
Is it a 1989 or a 1998?

I will assume it is a 1989 until you say otherwise.

Do the injectors spray fuel when the engine is Cranking?
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Old 04-03-2011, 10:22 AM   #5
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Re: 89 tbi running & starting problem

sorry its a 89! and the injectors are spraying when i am cranking.
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Old 04-03-2011, 10:34 AM   #6
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Re: 89 tbi running & starting problem

It sounds like the truck has more then one problem.

It could have a bad TPS. More then one symptom points to the TPS.


If the ECM does not see a voltage reading on the TPS that shows it is at idle, the ECM will not move the IAC.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.


*************

TPS testing for TBI


TPS Throttle Position Sensor testing:
Piercing the wires to do testing can cause future problems.
When I can’t use a scan tool, I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)

With the key on and engine off:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

Many people just test it with the engine off using an ohm meter on the terminals of an unplugged sensor to test for a smooth change in resistance as the throttle is moved.
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Last edited by ChevyTech; 04-03-2011 at 11:14 AM. Reason: fix poor wording
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Old 04-03-2011, 11:21 AM   #7
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by ChevyTech View Post
It sounds like the truck has more then one problem.

It could have a bad TPS. More then one symptom points to the TPS.


If the ECM does not see a voltage reading on the TPS that shows it is at idle, the ECM will not move the IAC.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.


*************

TPS testing for TBI


TPS Throttle Position Sensor testing:
Piercing the wires to do testing can cause future problems.
When I can’t use a scan tool, I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)

With the key on and engine off:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

Many people just test it with the engine off using an ohm meter on the terminals of an unplugged sensor to test for a smooth change in resistance as the throttle is moved.
I did the cleanup on the IAC and checked the TPS but I will check them again. As for the fuel pressure regulator that should affect the fuel pressure so I don, think it is that.
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Old 04-03-2011, 07:31 PM   #8
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Re: 89 tbi running & starting problem

Ok checked the TPS and re checked the IAC both are good. I did disconnect the map sensor and the truck didn't change the idle at all. When I did fire it up today it ran smooth but when I mashed the throttle it almost died till I let off, but when I did it again had a nice fire ball as it back fired through the throttle body. "On a funny note it scared the crap out of my buddy that was over!"
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Old 04-03-2011, 07:57 PM   #9
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Re: 89 tbi running & starting problem

Was the fuel pressure tested while the engine was running? How was it tested?

When you disconnected the MAP sensor, did it set a trouble code?

Is there a good vacuum supply getting to the MAP sensor?

Did the problem start all of the sudden, or was it a slow deterioration.

Have you checked the timing?

Have you removed the distributor?
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Old 04-03-2011, 08:06 PM   #10
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Re: 89 tbi running & starting problem

I bought the truck it did not run the engine harness was cut so I replaced it. the fuel pressure was tested wile it was running and wile it was not. I spliced in to the main fuel ling by the filter. did not check for a code this time but last time it did set a code and it has great vacuum supply. The timing is computer controlled and no i did not remove the distributor.
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Old 04-03-2011, 08:18 PM   #11
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Re: 89 tbi running & starting problem

Check the timing and make sure you unplug the set timing connector when you check it.
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Old 04-03-2011, 08:54 PM   #12
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Re: 89 tbi running & starting problem

what set timing connector if I unplug anything from the distributor it dies.
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Old 04-04-2011, 10:18 AM   #13
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Re: 89 tbi running & starting problem

You need to do some reading to get up to speed on TBI trucks.

There is so much good information available on this forum. Use the search feature.

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Old 04-04-2011, 10:41 AM   #14
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by maynardtruckin View Post
I did the cleanup on the IAC and checked the TPS but I will check them again. As for the fuel pressure regulator that should affect the fuel pressure so I don, think it is that.
Have you checked your coil? I had a bad one on my 95 that made it buck, kick and studder untill I did a complete tune up including the coil now it runs like new.
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Old 04-05-2011, 11:10 AM   #15
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Re: 89 tbi running & starting problem

CT,

Bob here (Little Red Truck). Mine is the 2wd, Chevy C3500, L80E, 7.4L, TBI that you fixed last year with the EGR switch.

Current issue is starting and low throttle miss-fire (45-50 mph). All plugs, wires, cap, rotor are new. No codes from Snap-On scanner. Scope showed good voltage thru wires. Snap-On could not find gear selected (Park) and searched P-L1 continuously. Cable that indicates gear selected in dash is broken. Contacts in P-R-N-D switch on column were cleaned an lubed with Di-electric grease. Ran compression check: 153,166,146,158,152,144,152,152 (1-8). Blocked EGR vac line and verified EGR open/close.

Nothing changes miss-fire.

Any ideas on where to go next?
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Old 04-05-2011, 11:22 AM   #16
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by Little Red Truck Tha View Post
CT,

Bob here (Little Red Truck). Mine is the 2wd, Chevy C3500, L80E, 7.4L, TBI that you fixed last year with the EGR switch.

Current issue is starting and low throttle miss-fire (45-50 mph). All plugs, wires, cap, rotor are new. No codes from Snap-On scanner. Scope showed good voltage thru wires. Snap-On could not find gear selected (Park) and searched P-L1 continuously. Cable that indicates gear selected in dash is broken. Contacts in P-R-N-D switch on column were cleaned an lubed with Di-electric grease. Ran compression check: 153,166,146,158,152,144,152,152 (1-8). Blocked EGR vac line and verified EGR open/close.

Nothing changes miss-fire.

Any ideas on where to go next?
I had a 97 K-3500 had a miss couldn't figure it out , the guy I sold it to took it in and they found the fuel regulator was bad but that was a Multi port. My 95 350 TBI was doing the same thing It turned out it was a weak coil.

Any one tell me where the fuel regulator is on the TBI models my 92 always stinks like gas real bad?
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Old 04-05-2011, 12:08 PM   #17
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Re: 89 tbi running & starting problem

Quote:
Any one tell me where the fuel regulator is on the TBI models my 92 always stinks like gas real bad?
The fuel pressure regulator is in the TBI unit.



Here is a link if they don’t allow hot linking.
http://www.snflupigus.com/Partspics/image054.jpg


Here is another illustration:
http://chevythunder.com/gm_throttle_...ction_pg_1.htm
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Old 04-05-2011, 12:19 PM   #18
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by Little Red Truck Tha View Post
CT,

Bob here (Little Red Truck). Mine is the 2wd, Chevy C3500, L80E, 7.4L, TBI that you fixed last year with the EGR switch.

Current issue is starting and low throttle miss-fire (45-50 mph). All plugs, wires, cap, rotor are new. No codes from Snap-On scanner. Scope showed good voltage thru wires. Snap-On could not find gear selected (Park) and searched P-L1 continuously. Cable that indicates gear selected in dash is broken. Contacts in P-R-N-D switch on column were cleaned an lubed with Di-electric grease. Ran compression check: 153,166,146,158,152,144,152,152 (1-8). Blocked EGR vac line and verified EGR open/close.

Nothing changes miss-fire.

Any ideas on where to go next?
I think I actually remember you. If I remember correctly you are a can-am race car guy.

A light throttle misfire at that speed is often caused by a secondary ignition leak, but you changed out all those parts.

It could be a lean misfire caused by a vacuum leak or low fuel pressure.

A dirty fuel filter can cause low fuel pressure.

Try the easy things first because the TBI trucks have no fuel pressure test port and the pressure should be checked with the engine running.

Make sure the distributor shaft will not wiggle sideways.

Instead of tacking onto this thread, new topic threads should get started for other topics so there are no misunderstandings.
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Old 04-05-2011, 06:34 PM   #19
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Re: 89 tbi running & starting problem

well I unplugged the timing set wire and after 6 times it finally stayed running to set the timing. Now after resetting the timing it still does the same. I also tried the coil and it didn't change. I am lost!
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Old 04-05-2011, 07:52 PM   #20
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Re: 89 tbi running & starting problem

Quote:
I bought the truck it did not run the engine harness was cut so I replaced it.
Being you bought it without it running; you should test the compression on all cylinders. It could have a mechanical engine problem causing a dead cylinder or cylinders.

A bad IAC can cause starting problems. After the engine is shut off, and before it is started, the IAC should open somewhat. When the engine is cranked, and if the IAC does not open at all the engine will usually “load up” or “flood”.

If the engine ran well after driven down the highway for a few miles, then it would sound like an IAC problem.

If it gets fouled plugs, and makes it into closed loop operation while still misfiring, the system will go very rich because of the constant misfire. When this happens the truck will run better with the oxygen sensor unplugged.

Did you splice wires or just replace harnesses?
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Old 04-05-2011, 08:02 PM   #21
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Re: 89 tbi running & starting problem

It has 125- 135 on all cylinders just put a new IAC in and all the harness needed was the fuel pump wires replaced.
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Old 04-05-2011, 08:23 PM   #22
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Re: 89 tbi running & starting problem

IAC passages can get plugged up with carbon or muck. Did you clean them?


The IAC (idle air control) is what gives the fast idle speed when started cold.


The engine should start well without the gas pedal being pushed down at all.


My guess is the truck runs so poorly you can not tell if the IAC is working or not.
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Old 04-05-2011, 09:24 PM   #23
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Re: 89 tbi running & starting problem

the IAC passages are clean and the IAC wiring and controller tests fine. As for what the IAC does I know. I also have replaced the hole throttle body with a new one.
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Old 04-06-2011, 09:37 AM   #24
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Re: 89 tbi running & starting problem

It has good fuel pressure, you tested the TPS, you replaced the ECM, IAC, TBI unit & coolant sensor, and it will not start without spraying starting fluid in it.

Have you looked at the spark plugs to see how they look? Are they Black? Are they clean?

Possibilities:
You replace the wrong coolant senor
The fuel pressure really low
The injectors are wrong and it is not getting enough fuel
The distributor is very worn and reluctor is hitting the points on the pickup
You installed the wrong ECM
The ECM has the wrong PROM in it.
There is a problem with the wire harness
Bad grounds between engine – body – battery
The plugs are so dirty it can’t possibly run right.
The engine has a mechanical problem – that compression is low
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Old 04-06-2011, 09:43 AM   #25
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Re: 89 tbi running & starting problem

Quote:
Originally Posted by maynardtruckin View Post
the IAC passages are clean and the IAC wiring and controller tests fine. As for what the IAC does I know. I also have replaced the hole throttle body with a new one.
If all else fails take it in to a good garage with all the test equipment. I know this goes against the grain of most of us but some times in the long run it's cheaper.
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