1989 Chevrolet gas pickup engine problem

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
This is about as far from a VW tdi as you can get, but maybe someone here can help. I recently bought a 1989 Chevy 3500 with the giganto 7.4 liter gas engine. It runs OK at highway speeds but at 35-40 it acts strange. It sort of put-puts, almost as if it's running out of gas, even if I depress the accelerator, but it will maintain speed at what feels like reduced power. Then, after maybe 5 or so seconds, it responds and accelerates. The check-engine light, if there is one, is not lit, and I think this predates OBD II anyway. What does this symptom sound like? I assume it has some sort of electronic/computer control. Could it be the controller? A sensor? Does this engine use a throttle position indicator?
TIA
 

El Dobro

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Feb 21, 2006
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NJ
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2017 Bolt EV Premier, 2023 Bolt EUV Premier
Unless someone removed it, it should have throttle body injection. Scan it with an OBDI scanner. It could also be the fuel pressure.
 

bam_bam_dip

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Nov 17, 2004
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Belton, TX
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99.5 Jetta TDI GL
El Dobro said:
Unless someone removed it, it should have throttle body injection. Scan it with an OBDI scanner. It could also be the fuel pressure.
A glorified fuel injected carb.
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
It definitely seems fuel related. I guess I could get AutoZone or someone like that to scan it for me.
 

VWWV

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Florida and WV
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passat 96, candy white
My old 89 5.7 (small block 350) had a throttle body injection system. I never experienced your problem but I would look at the fuel filter (located under the passenger side rail about mid-way back) and the distributor rotor and cap. Plug wires could also be bad? There is a check engine light below the speedometer (reads: Service Engine Soon). Mine lit up when the oxygen sensor needed replacing every several thousand miles. Best gasser engine ever built, IMO, and the 7.5 is right there, also. You might spray some carb cleaner down the throat of the TBI to clean some of the junk out. I used to do this regularly.
 

catmandoo

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ia
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2000 jetta gls tdi,91 2dr jetta gl n/a diesel
i'll second the cap and rotor.i had a similar experience with my 94 dually it would just fall on it's face then all the sudden pick right up like nothing happened.fuel filter couldn't hurt either as most of em get neglected.
 

mavapa

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Joined
Aug 29, 2000
Location
rome, ga
TDI
2001 golf
I have a fuel filter (plus air filter - man was it dirty!) and I'll put it on this weekend. I didn't think about cap and rotor or other electrical problems because it seemed so fuelish, but I'll check that, too. Coil also. Thanks.
 

PoochiePD

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Mar 23, 2005
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Sun Prairie, WI
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2013 Tempest Blue Jetta
Does it happen every time you reach that speed?
I had an '89 with the 5.7L. I never really pinpointed the problem, but it was an intermittent problem that dealt with the wires from the ECU, which was mounted behind the glove box, where they were run through the firewall. There should be a big bundle of wires under the dash that you can reach without removing anything. Whenever it would act up, I would just reach down and wiggle that bundle and it would usually "fix" itself. Most of the time it would be fine, other times it would frequently act up. I think it's either a positive or a ground for one of the injectors. (I unplugged one injector at a time as it was acting up once while someone else kept it running. Only one was working.) It would not idle and would run really rough. Pretty embarrassing when you're on a date and it does it. "Could you, uh, wiggle the bundle please? Thanks." :cool:
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
Well, I fixed the problem, if by "fixed" you understand me to mean "the truck's engine won't run." I replaced the distributor rotor and cap, and it started but ran roughly. Then I rechecked the plug wires and firing order and found a problem with wire-to-plug order, which I "fixed." Then it wouldn't start. I checked and rechecked the firing order, made sure I had a spark on No. 1 plug, checked the position of the rotor at TDC for No 1 (yes, on compression), and the engine spins but won't even hint at firing. The manual gives the firing order (18436572, with No 1 cylinder at right front facing the engine, and No 1 plug at 6:30 on the distributor, with 12 at the rear and 6 at the front), and every online source gives the same. I have looked at the plug wire order a dozen times and it's what the manual says. Logic would indicate that what I did caused the problem, but I cannot identify anything wrong with the ignition at this point. Now I remember why I hate working on gasoline engines.
 

mavapa

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Location
rome, ga
TDI
2001 golf
DPM, that is actually a very good question. The manual and the online sources say clockwise, and that is what I have assumed. However, I have not verified that to be the case.
 

TDIJetta99

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Port Jervis, New York, USA
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03... Faster than yours =]
The 454's(7.4) have the same distributor and firing order as the smallblocks(5.0, 5.7, etc). Same ECU wiring for the throttle body injection too.. I've had a few of these over the years (the 7.4 that powers my camaro came out of an 89 pickup). Take the number one plug out and bring it up to the compression stroke for #1.. Then take the cap off and see where the rotor is pointing.. Someone may have had the distributor out at some time and dropped it in in a different position than the manuals say.. The firing order is still the same, just start from whatever post the rotor is pointing at... The distributor turns clockwise.. Cylinders 1,3,5, and 7 are on the driver's side and 2,4,6, and 8 are on the passenger's side... Also if you're sure you have spark and the wires and everything are on right, take the air cleaner off and crank it.. You should see a good spray of fuel coming out of the two injectors.. If not you need to check for fuel pressure.. 9-13psi is what you'll be needing... They will run on as little as 4-5psi, but they'll run reall crappy... The 7.4's always seem to run a little on the lean side part throttle and that will get a lot worse if the fuel pressure goes a little low for some reason...

Also, don't waste your time trying to scan it... It's pre-obd2 so it won't really help you much, especially if it doesn't start... One way to see if the light works is to just turn the key to the on position, the light should be lit...

Hope some of this helps...
Jason
 
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mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
Last weekend I tried some starting spray and had no firing, so I am pretty sure it's electrical. That makes sense, since that's what I changed. I checked and was getting a spark to the plug at No. 1. I have checked and rechecked the firing order and spark plug wires. I have been assuming clockwise rotation. I also looked at the rotor when No. 1 was at TDC, and it appeared to be pointing at what I had been assuming was the No. 1 plug. But it still won't fire. I'll be sitting in the engine compartment next weekend trying to figure out what I've done. Thanks for the help.
 

VWWV

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Sep 9, 2003
Location
Florida and WV
TDI
passat 96, candy white
quote: "I'll be sitting in the engine compartment......". That is one of the benefits about working on those engines. Put your butt on the radiator fan shroud with one foot on either side of the engine. Helps if you take off the hood - less strain on your back. Hope you find it.
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
Well. It seems that the distributor might have been installed a little off. Everything I read said No. 1 is at 6:30 on the distributor (12 is at the rear of the engine), so that's the way I installed the plug wires (stupid me, I should have noted the plug wire positions on the old distributor cap). Anyway, I rotated every wire one position clockwise and the engine started and ran better than it did when I got it.
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
I spoke too soon. The truck runs again, but it still has what seems like a fuel delivery problem. It runs OK and then runs as if it's starving for fuel. Then, after a short time, it picks up again and runs OK. It's almost as if it's only running on half its cylinders. I plan to replace the fuel filter, but I don't really see how that could be the problem, since it's intermittent.
 

TDIJetta99

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May 17, 2005
Location
Port Jervis, New York, USA
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03... Faster than yours =]
I still think you should check the fuel pressure... Take it for a ride with the gauge attached....

edit: I just remembered, I had one of these trucks with the 5.7 that gave me some trouble with a hesitation like yours... After a few times test driving the truck with the scanner, I noticed the oxygen sensor voltage was pegged around 1.5volts, the most the sensor will produce is about 1volt... AFter changing the sensor, tnere was no difference.. Turned out that the exhaust system wasn't grounded anymore, causing the sensor to give a false signal... When the ECM sees a high O2 sensor voltage it will lean the mixture out because it thinks the engine is running full rich...

Try taking a small piece of wire and grounding the exhaust somewhere near the sensor and see what happens.. If the sensor is installed in the manifold I would double check all the grounds in the engine compartment, especially the ECM grounds on the back of the passenger side cylinder head...
 
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mavapa

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Aug 29, 2000
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rome, ga
TDI
2001 golf
Well, I finally decided I don't have enough time or the use of both arms, so I took it to a shop. There, I hope, they can fix this truck so it will run reliably enough to haul the various types of money sinks I get at Lowe's.
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
Problem solved

For the sake of completeness, here is what the shop found. A previous owner had plugged a vacuum line from the throttle body with a short section of vacuum line with a bolt in the end. That line was leaking, and the MAP sensor thought the engine needed more fuel, so it was running very rich. They fixed that and it seems OK, although I think it's still going to have a very bad drinking problem.
 

mavapa

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Aug 29, 2000
Location
rome, ga
TDI
2001 golf
... and then a ball joint disintegrated and the front end collapsed onto the front wheel. We got that repaired the same day, and then the driver side window came out of the track and won't roll up. I plan to see what I can do to fix that with one hand tied behind my back (actually, with one arm in a sling - rotator cuff repair).
 
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