1989 Chevy Suburban 350 - stalls when in forward or reverse while breaks are engaged for a while. TBI problem?

MIKECBECKER

MEMBER
1989 CHEVROLET SUBURBAN
5.7L
V8
4WD
MANUAL
34,000 MILES

This is a little long winded -sorry. I recently purchased a 1989 Chevy Suburban 350 with 33,000 original miles. It was last registered six years ago. When I bought it, it was on jack stands and had an inch of dust on it. Full tank of old gas too. The previous owner said before he put it up on jack stands he did a full and complete tune up. After going through the basics, inflating the tires and charging the battery, it started right up and a/c blows cold air. I drove it 150 miles to my home. On the way home it stalled a few times when stopping at lights or to pay tolls, but started right back up every time. I’ve been driving it. Put about 300+ miles on it - ran through the old gas. When in drive or reverse with my foot on the brake, the truck would stall, but would idle smooth in park or neutral. At lights if I put it in neutral it would be fine. The truck ran awesome (expect for the stalling issue). Transmission shifts perfectly. There are no pending codes and the check engine light is not on.

I tried cleaning the TBI with injector cleaner, while the TBI was attached to the intake manifold - no real help. I changed the fuel filter - the fuel filter was not blocked, but nasty stuff came out of it. Changing the fuel filter had no real effect.

I read a bunch of threads on the net about the stalling problem while in gear. I changed the IAC - there was a slight improvement. After changing the IAC it took a few more seconds, when in gear, before it would stall. I changed the EGR and PCV - which had no real effect. I changed the coolant temp sensor - also no effect.

The next thing seemed to be the fuel pressure regulator. The kit for the pressure regulator and the entire TBI rebuild kit with a new pressure regulator was a nominal difference, so I figured I might as well get the rebuild kit. I disconnected the TBI from the intake manifold. The gasket between the TBI and the intake manifold did not seem right. The gasket seemed to have squished into the underside of the TBI into the channels/canals that lead to various vacuum lines. Parts of the TBI were pretty dirty. Where the IAC goes into the TBI was pretty black. The vacuum lines were pretty dirty, but nothing was blocked by any means. I cleaned the TBI with throttle body cleaner and followed the instructions and did the rebuild (and changed the pressure regulator). It was not overly complicated. I don’t see how I did anything wrong.

When I put the TBI back on the intake manifold, reconnected everything and started up the truck it idled really high and steady and never dropped down. Any thoughts? I would like to first find out what may be going on with the TBI and if I did something wrong in the rebuild. Also the check engine light is not on and there are no pending codes.

3 Replies

CARADIODOC

MECHANIC
33,742 POSTS

I may have to enlist the help of another expert, but the idle speed motor is the same part used on Chrysler products. Remove that unit from the throttle body, then you'll see a gold-colored "pintle" valve. You'll have to tug rather hard, but you can pull that valve out by hand. The more it's extended, the less air it will allow to bypass the throttle blade, and the lower the idle speed will be.

If that valve is already extended or doing so doesn't lower the idle speed, there is some other vacuum leak. If you can't find a loose hose or a problem with the gaskets, inquire at an auto parts store that rents or borrows tools if they have a smoke machine. That will allow you to inject a white, non-toxic smoke into a vacuum port, then you can watch for where it sneaks out.

These motors have a very low failure rate, at least electrically, and that's what would be detected by the Engine Computer which would set a fault code. The computer won't detect a mechanical problem with it though. If you leave that motor out but connected to the wiring harness, you should see that valve move in and out in preparation for starting each time you turn the ignition switch on and off. If it doesn't move, it is likely bound up from sitting for so long. That would likely explain the intermittent stalling, which, in this case, is idle-speed-related. The stalling is not related to a component failure since the engine doesn't stall at highway speed. That idle speed motor is out of the picture at that time.

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Thursday, July 23rd, 2015 AT 11:12 PM

MIKECBECKER

MEMBER
2 POSTS

Thank you so much for the quick response. Your response is very logical, but unfortunately I do not believe there is an idle speed motor applicable to the TBI on my 1989 Chevy Suburban 350. There is an idle air control valve, which I replaced and seems to be working fine. Do you think the fuel pressure regulator could be the problem?

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Friday, July 24th, 2015 AT 6:28 AM

CARADIODOC

MECHANIC
33,742 POSTS

"Idle speed motor" and "idle air control valve" are the same thing. Different name; same part. What confuses some people is the term, "motor". It is not a DC motor with brushes that spins rapidly. It is a "stepper" motor that uses four coils of wire around a magnet. The Engine Computer pulses those coils with varying voltage and polarity to place the armature at the desired position. As that armature rotates, it spins a threaded shaft that extends or retracts. That shaft moves the pintle valve to expose more or less of the air passage that bypasses the throttle blade. At the same time, the computer varies the percentage of time it pulses the injectors on to adjust how much fuel goes into the engine. Varying the amount of fuel and air is how it controls idle speed.

The coils of wire inside that unit are rather tough, which is why electrical failures are very rare. Also, the Engine Computer has no way of knowing where that valve is positioned. It only knows which "step" it has commanded the unit to go to, and it assumes that is what took place. If the idle speed is still too low, it just keeps increasing the number of steps it's calling for. A failure to get the desired result, meaning the idle speed didn't come up to where it should be, could be due to a gummed-up motor shaft, an air passage blocked with carbon build-up, a misfiring spark plug or injector, ... Things like that.

What IS monitored by the computer is the current flow to the coils in the motor, so that is what will cause diagnostic fault codes to set. Most people jump on the motor right away as the cause and replace it, then there's a 50 / 50 chance the problem clears up. You must be aware that a fault code never says to replace a part or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. When a part is referenced in a fault code, it is actually the cause of that code about half of the time. It's just as likely you'll find a wire rubbed through and grounded out, it's cut, or a connector terminal is corroded. The 50 percent chance a new motor solves the idle speed problem is there's usually a corroded terminal in the connector. The motor didn't have to be replaced. Simply sliding the connector off and back on scratches a new shiny spot to temporarily make a good connection.

In the case of the idle speed motor, the percentage changes to probably 90 percent there is an electrical problem outside of it.

I have never heard of a relearn procedure having to be done on a GM product, but on a Chrysler product, when the battery is disconnected or run dead, the Engine Computer has to relearn "minimum throttle" before it will know when it must be in control of idle speed. Until that is done, idle speed will be too low and cause hard-starting and stalling. To cause the relearn to take place, you simply drive at highway speed, then coast for at least seven seconds.

Also, to give you an idea of the control the computer has over idle speed, I watched a trainer from Chrysler disable six injectors on a V-8 Jeep engine, and the idle speed was still perfect. Obviously it didn't run very well, but it ran.

Fuel pressure is not much of a problem on throttle body injection systems. They commonly only run at around 14 psi. There were some problems with loose connections on the injector plugs, but it's more likely that would show up as a sudden loss of power while driving, especially from road vibration.

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