Hesitation and stumble on my 1988 4.3L

Jerther · August 27, 2005

I decided to start a new topic, dedicated to my trouble, following this topic:

http://www.gm-trucks.com/forums/index.php?...t=entry318563

So heres the problem:

Cold or hot, at any time, whatever the weather conditions, during acceleration, I feel hesitation and stumble. When the gas pedal is pressed a little bit, everything is ok. The hesitation appears when I press further, until I get to the floor. On low rpms, sometimes the engine almost stops.

Here's what I changed with no change to the trouble:

- TPS

- EGR Valve

- Rotor and Dist cap

- Spark wires

- Ignition module

- Ignition coil

This is what I tested:

- Fuel pressure: 14 PSI STEADY.

- Fuel injectors resistance: 1.3 to 1.4, and got that resistance on 4 other injectors.

- Code in the computer: none

- Exaust is always ok. I never saw any black smoke.

- The 2 holes in the intake manifold for the EGR Valve are clean, and no apparent carbon buildup in the barrels.

- O2 and Map sensors seem to work fine

The only thing that did something was timing. It was first set to aprox. -4. The hesitation was awful. Got it to +10 and the hesitation almost completely dissapeared. Then get it to +12. Nothing changed really except for BAD mpg. Those setups were by mistake since i didn'T see anything on the timing plate. So i cleaned it, and got the engine timed to 0 exactly. On 0, the hesitation gets back. Heard no pinging/knock at any of the tested timings.

Now, I could time the engine at +6 or something, like i've been told once. But, the emission sticker clearly says "Timing: 0 degrees". Unless i'm given a good reason, I'll believe the problem is elsewhere.

My quest is now on the ignition coil. I've got a spare one so I won't have to buy one. But, this one is for a V8 engine. 5.0 or 5.7, I don't know. Will it work on my V6? I don't want my engine to blow up =)

So my 2 questions are: Would that coil work on my engine, and do you have any clue of what the problem would be?

edit: the coils for the 5.7 and 4.3 are the same.

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Here's a Log of what I changed and checked on my engine:

- TPS

- EGR Valve

- EGR Valve Gasket

- Rotor and Dist cap

- Spark wires

- Ignition module

- Ignition coil

- Fuel pressure: 14 PSI STEADY.

- Fuel injectors resistance: 1.3 to 1.4, and got that resistance on 4 other injectors.

- Code in the computer: none

- Exaust is always ok. I never saw any black smoke.

- The 2 holes in the intake manifold for the EGR Valve are clean

- O2 and Map sensors seem to work fine

- Removed the catalytic converter

- Timing to +X helps. (Currently +6)

mmmikkke · August 27, 2005

I didn't see an 02 sensor in your list. Has it ever been replaced?

The hesitation sounds like a lean-out. Maybe the O2 sensor is telling the computer there's oxygen there when there isn't. So the computer doesn't enrich the mixture...and you get a lean stumble.

Jerther · August 27, 2005

The O2 sensor was replaced a year ago. I'll check this out though.

mmmikkke · August 27, 2005

If when you full throttle the stumble is corrected, it's not your coil.

Jerther · August 28, 2005

here are some graphics. The numbers are all aprox and aren't the actual values. The goal here is to illustrate the torque feel.

I found tonight, that the hesitation is still present on full throttle, as shown on the left graphic.

I also found that the hesitation is harder to feel on higher rpms.

The expected torque is shown here according to the torque I got when the timing was set to +10.

Jerther · August 29, 2005

I read about the O2 sensor. To work, it has to reach an operating temperature of about 600 degrees (C or F, don't know). The engine uses the infos from the sensor to change the injector pulse width, so it can control the air gaz mixture and keep it at 14.1:1 (something near that)

BUT, as the sensor is under its operating temperature, the computer works in open loop (doesn't care about the O2) and tries to blindly control the mixture.

however, the hesitation happens even if the computer is in open loop.

Also, I guess a faulty O2 sensor would generate a code in the computer.

This is, however, just my opinion...

I'll try this:

Looks like you've covered the basics. I'd get the O2 sensors checked. It does sound like it's leaning out. You could check the plugs after a hard acceleration run. See if they look lean. Do a hard accel run then kill the motor. That will give a look at the plugs as you were accelerating.
Be careful, though. With the motor shut off, you have no power steering or brakes.

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Jerther · August 31, 2005

I think I have two more clues here...

Early in the day, when the engine is cold, I can hear some light knocks when accelerating slightly. But it goes away after about 3 or 4 seconds. I got my engine timed to about +6 as a temporary fix for the hesitation. Maybe that's too much?

The hesitation always appear at about the same Throttle position. I flipped my air cleaner cover upside down a couple of days ago so i can hear my engine roar

I also hear a whistling sound beggining at that particular throttle position. The sound of the air going through the throttle plate which I know is normal, but... hmmm... Vacuum... soooo that points to the MAP... It's worth a quick check.

Here's what I plan to check:

O2 sensor

MAP sensor

Spark Plugs for some lean out signs

Ignition Coil

I also plan to build a cable that will connect my laptop to the ALDL connector, hoping to see the "Rich/Lean" data from the ECM. Check this out: http://www.techedge.com.au/vehicle/aldl8192/8192hw.htm

For now i need to know if an ignition coil took from a 5.7L will work on my 4.3L... Googled on that for a couple of hours already.

Jerther · September 3, 2005

Okay, I made an ALDL cable to connect my laptop to my ECM. It works pretty well

My engine knocks... I don'T know if it's normal, and it not audible, but it knocks. Not all the time though. It knocks about 10 times (knock count) on acceleration mostly.

And another thing. The O2 varies from 0.1 to almost 1, even at idle.

I can tell the engine starts in open loop, then switch to closed loop, according to the flags.

The Asychronous Fuel flag is always on and the engine tends to run mostly without the Rich flag on, especially during acceleration.

I'm using WinALDL software.

Jerther · September 13, 2005

I had an ignition coil at hand so I changed it with no change at all.

O2 and MAP sensors seem to work fine.

Next steps are the removal of the catalytic converter and some tests with the Knock sensor.

I bought a new battery for my laptop so it can run longer that 10 minutes ;)

So I did a few runs with WinALDL running. Everything seems ok but two things: the "Asychronous Fuel Flag" is always on and I get these knock count results for a 30 minutes run:

I guess this is a lot of knock! I'd like your advice on this. Could it be that the knock sensor sends a knock signal when it shouldn't, causing a timing retard and thus hesitation?

And I'd really like to understand why, when I advance the timing, the hesitation partly goes away??? At 0 degrees, the hesitation is awful.

manders500 · September 22, 2005

I have the same issue with my 1999 Sierra K1500. Fuel pressure is 58 lbs, vaccum is a steady 17", had the injectors drip cleaned, plugs look good, cleaned the throttle body and banged my head on the wall a few times.

Did you find anything yet?

I had an ignition coil at hand so I changed it with no change at all.

O2 and MAP sensors seem to work fine.

Next steps are the removal of the catalytic converter and some tests with the Knock sensor.

I bought a new battery for my laptop so it can run longer that 10 minutes ;)

So I did a few runs with WinALDL running. Everything seems ok but two things: the "Asychronous Fuel Flag" is always on and I get these knock count results for a 30 minutes run:

I guess this is a lot of knock! I'd like your advice on this. Could it be that the knock sensor sends a knock signal when it shouldn't, causing a timing retard and thus hesitation?

And I'd really like to understand why, when I advance the timing, the hesitation partly goes away??? At 0 degrees, the hesitation is awful.

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Jerther · September 23, 2005

The only thing I found that really changes something is the timing. I set it to +6 and it partly goes away.

I got my catalytic converter off earlier this week. Didn't change anything but the sound

I also replaced the egr valve gasket with a new one. It was hard and glazed.

I also checked the EGR Holes on the intake and they are clean.

I'll edit my original post to reflect this week's updates...

manders500 · September 24, 2005

I think I am going to reprogram my ecu first and see how that goes. I really wish I had a carburetor instead of fuel injection for this issue. All of these sensors that influence the program decision makes this a nightmare. I am going to look more this weekend and if I find anything I will post back.

The only thing I found that really changes something is the timing. I set it to +6 and it partly goes away.

I got my catalytic converter off earlier this week. Didn't change anything but the sound

I also replaced the egr valve gasket with a new one. It was hard and glazed.

I also checked the EGR Holes on the intake and they are clean.

I'll edit my original post to reflect this week's updates...

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Jerther · September 24, 2005

About the ECU, I tried to reset it but that didn't change anything.

I'm thinking about the Electronic Spark Control (ESC) Module here. But I don't have a spare one to test...

leifcoburn · July 12, 2011

About the ECU, I tried to reset it but that didn't change anything.

I'm thinking about the Electronic Spark Control (ESC) Module here. But I don't have a spare one to test...

I would like to know if this problem was ever resolved. I have been through this exact senario, plus more with my 1988 G10 van.

only difference is it hesitates at idle. Ruuuuuuuuuuuuuuunn, sputter Ruuuuuuuuuuuuuuuuuun sputter

usaa · December 5, 2015

psi was 14? Was it checked in the line before or after the fuel filter? Try excelerating up a hill, if it dogs bad it could be your filter. heck a gm filter usually isn't too much and replacement can never hurt.