Nagging problem with 1999 Suburban

[littlecanoe]

We've owned a 1999, old body style, GMC  Suburban since 42K miles. It now has 179K on it. 
We've maintained it well but have had a nagging problem with the throttle. 
I'd like to pinpoint the problem.

The symptoms:  When driving there are times when you let off the gas pedal RPM's stay up and the engine keeps pushing.  If the brakes are applied you get the feel in that you are fighting the push of the engine.

Another newer problem is an intermittent tendency for the engine to stall upon starting.  It will crank over and start firing but sputter and die.  Start it again, keep the RPM's up for a bit and it runs fine then.



I've cleaned the Throttle Body and have replaced the Idle Air control valve.
No codes are showing up on the computer.

I'm hoping that this is a common 350ci problem for this generation GMC.  I'd like to get this cleared up.
Thanks for the help and ideas in advance,

lc

[mechanic]

This one needs to be hooked up to monitor the throttle position sensor, relative to the throttle motor which controls the butterfly in the throttle body.  Sounds like a throttle position sensor problem....but it may be mechanical linkage sticking as well, or a cable.  With the engine off, remove the breather and work the throttle by hand...it should give you an idea if the cable is sticking or "draggy".
 
When was the last time you changed plugs?  Believe it or not, the sputter you describe can well be just spark plugs.  With platinum plugs, you can run them until the electrode is almost gone, but the engine runs richer when cold and until it starts into closed loop, and the richer mixture needs a very hot spark.
 
Could be a couple hundred things, but these you can check easily.
 
Ben
 

[Old Syko]

I agree with Mechanic with one exception.  The throttle position sensor can be checked with nothing but an analog ohm meter.  Unplug the connector and attach the meter to the appropriate 2 connections and slowly open and close the throttle.  You should see a gradual and constant sweep of the needle without any glitches at all.  I don't remember the exact low and high readings you should see but the linear sweep is the important part anyway.  The TPS is nothing more than a potenteometer.


BTW a vacuum leak, especially around the intake can cause this also and can be a bear to locate.  A lot of times this can be discovered due to erratic heater control operation, believe it or not.

[Lloyd Smale]

my guess would be the same. Probably your throttle position sensor. Possibly could be your idle air control valve too. Thats an easy fix though as you can usually just pull them out and clean them.

This one needs to be hooked up to monitor the throttle position sensor, relative to the throttle motor which controls the butterfly in the throttle body.  Sounds like a throttle position sensor problem....but it may be mechanical linkage sticking as well, or a cable.  With the engine off, remove the breather and work the throttle by hand...it should give you an idea if the cable is sticking or "draggy".
 
When was the last time you changed plugs?  Believe it or not, the sputter you describe can well be just spark plugs.  With platinum plugs, you can run them until the electrode is almost gone, but the engine runs richer when cold and until it starts into closed loop, and the richer mixture needs a very hot spark.
 
Could be a couple hundred things, but these you can check easily.
 
Ben

[charles p]

With my luck, it would have two different problems.

[littlecanoe]

I appreciate the feedback fellas.  I'm a bit rushed this week.  Hopefully will be able to look at it over the weekend.

lc

[charles p]

Let us know what you find.  Nothing like learning from someone else, for free.

[Larry L]

I think you have separate issues. The engine speed when letting off of the gas and braking is a transmission torque converter issue. The converter is suppose to unlock when you apply the brakes. Sounds like that's not happening. Simple test. Take it out on the road and at hiway speeds apply a little more gas than required to keep it at speed. Apply the brakes at the same time. The engine speed should advance considerably. If you have a tach, it should go up a lot depending on how much gas you've given it- 800-1500 rpm.


In regards to the engine stall at cold start up. With the mileage you have on it the injectors are probably tired and leaking down overnight. I'd try some fuel system cleaner with either a Stoddard Solvent base like BG44 or a PEA like Techron. For a fuel system cleaner, these are the only ones that work so don't just buy anything thinking it will work. Most of these wonders in a can are nothing more than kerosene. I'd also check all of the vacuum lines on that 350 as it can also give you the same issues. If your PCV is anything but an AC Delco, that could be the problem especially if it's a Fram unit.

[littlecanoe]

If it's a torque converter issue would it happen all the time or could it be intermittent?

With some of the things that I've done already, it has improved but hasn't fixed the problem.

lc

[Rex in OTZ]

The funky anti skid brake problem (brakse wont grab when you wnat them to) is corrosion on the speed sensors mounted on the axel's

[.54]

I have a '99 Silverado, they are said to develop vacuum leaks around the intake gasket. Squirt some starting fluid around the intake gasket and if it revs up you might find a leak.

[tt4617]

With reference to the engine speed, I had a Buick Century with the exact same symptoms and it was the throttle position sensor.

[littlecanoe]

Well,  The low idle valve and throttle position sensor were replaced.  This helped a little bit.
I bought a used Throttle body and the problem disappeared.

What's tickling my brain is that upon studying the original throttle body I don't understand what was wrong with it.  Is this a spring wear issue or are there wear parts in there that I'm not seeing and taking into account?

lc

[mechanic]

Could have just been carbon build up in the throttle body, preventing the throttle butterfly from moving freely.  Don't throw away the old one, it could possibly be cleaned and reused later.
Glad you got 'er going.
 
Ben

[Lloyd Smale]

sometimes the bushings in the throttle body that on  throttle butteryfly shaft rides in can get sticky.

[cwlongshot]

sometimes the bushings in the throttle body that on  throttle butteryfly shaft rides in can get sticky.

I was just thinking this as well as simple wear at these bushings.

I am a firm believer in Sea Foam. Its like 10$ a can. I like 2-3 oz per gal for "problem" engines. Then 1-2 oz per gal for maintaince. Buy a can or three and try it. Generally speaking I am not a believer is snake oil type fixes but this stuff has general merritt and I use it in ALL my engines!! Great stuff!!

Google "Seafoam engine treatment"

CW

[Lloyd Smale]

problem in a high milage unit can even be that those bushings get worn out.

sometimes the bushings in the throttle body that on  throttle butteryfly shaft rides in can get sticky.

I was just thinking this as well as simple wear at these bushings.

I am a firm believer in Sea Foam. Its like 10$ a can. I like 2-3 oz per gal for "problem" engines. Then 1-2 oz per gal for maintaince. Buy a can or three and try it. Generally speaking I am not a believer is snake oil type fixes but this stuff has general merritt and I use it in ALL my engines!! Great stuff!!

Google "Seafoam engine treatment"

CW