Hesitation On My 4.3L 1988

[Jerther]

Hi! I've been looking to solve this problem for more than a year now. I gathered many information here and there, some on the net and some from mechanics. From neither I found an answer.

 

But most of the infos I've got are from my own tests. Here is a detailed description of my truck's behaviour:

 

No matter the temperature and the weather. Cold (open loop) or hot (closed loop). The engine Hesitates (leans out, low O2 value) when accelerating. But furthermore, when I lift my foot a little from the pedal, the engine gains some power (enrich, higher O2 value). Then if I stop lifting my foot, the engine looses some power again until I lift it again, or after a short time where I see the O2 sending a higher value.

 

When my engine does this, I put it to the floor then I lift my foot gradually so I feel like I get the most I can get. This isn't very economical... But sometimes, once the pedale is to the floor, I feel the engine gradually loosing some power for a second or two, and gradually gain it back!

 

If I don't let it warm up on a cold day (near freez point), and just go, Everything seems fine. I feel good power and no hesitation. But after a couple of minutes, the hesitation appears.

 

When I clutch at low RPMs, in the worst case the engine almost dies. I sometimes get caught at stops and lights...

 

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

 

- TPS

- EGR Valve

- EGR Valve Gasket

- Rotor and Dist cap

- Spark wires

- Ignition module (the one inside the dist. Cap)

- Ignition coil

- Removed the Catalytic converter

- Throttle Body and its gasket, but kept injectors and regulator.

- Coolant Temperature Sensor. Old one was a bit off.

- PCV Valve.

- Thermostat.

- Fuel Body (Injectors and a new regulator)

 

- Two bottles of Penzoil Injector Cleaner additive

- Ran a bottle full of Kleen-Flo Combustion Chamber Kleen through break vacuum, pcv valve and directly into throttle body.

 

This is what I tested/checked:

 

- Disconnect the EGR Valve and plug the vacuum hose

- Disconnect the EGR Valve Solenoid + above

- Fuel pressure: 14 PSI STEADY. but i would have to chekc this again...

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

- No code in the computer

- Exaust seems to be always ok. I never saw any black/blue smoke.

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

- There is some carbon on the driver side of the intake.

- O2 and Map sensors seem to work fine

- Platine Spark Plugs are ok (checked 3 of them)

- O2 indicates a very lean mixture during hesitations

- Happens also when in ALDL mode.

- In cold weather, happens more seriously when the engine has been running for a couple of minutes.

- Lowest idle speed (after warm up, either open or close loop) is a bit rough. The engines RPM varies from 500 to 650 RPM and vibrates a bit.

- 4.97V at Coolant Temperature Sensor and TPS connectors.

- ECM accurately reads values from Coolant Temperature Sensor resistance.

- Did a compression check: between 145 and 165 after 4 revs

- Checked spark plugs: seem all ok except for some additive deposits I used...

- No vacuum leaks around the intake manifold

- A bolt or two were loose on the intake, tightened them and no change

 

While looking for vacuum leaks, I found a leak at the exaust manifold, at the joint right after the O2 sensor...

 

Advancing the timing helps a little. When tired of testing, I run at about +6. And this is the only thing I found that helps.

 

I run at about 12-13 MPG (3.78 Liters per Gallon).

 

I have a ALDL connector to connect my laptop to my ECU if needed.

 

Now, i'm sure i'm facing a lean out problem. When in closed loop, and when hesitating, my O2 tells me the mixture is way too lean. O2 says it gets richer as the hesitation goes away.

 

Thank you!

 

Edit: Updated and cleaned up the post a little bit.

Edit: Added info about the temperature at which the problem occures

Edit: Added infos about Idling, since the new throttle body lowered the idle speed.

Edit: Updated Gaz Mileage

Edit: Added new infos about coolant temperature sensor.

Edit: Added PCV Valve. This one lowered idle speed even more. Also added Thermostat which raised the operating temperature a bit.

Edit: addes info about compression check and vacuum leaks

[Jerther]

I did a test run 5 minutes ago with my laptop plugged to the ALDL.

 

First, I tried the ALDL mode (10k resistor). This is supposed to disable the timing control. Well the hesitation is still there so the problem is not in the timing. Well this is only my opinion, I may be wrong...

 

As for the collected data. BLMs range from 114 to 128, and some times is at 144-145. It is mostly around 120. I've read that is should be around 128 all the time... Low values mean a lean mixture, right?

 

Next, I get a great load of Knocks... Timing @+6 I guess...

 

But the main clue I found is the readings at the O2. When I accelerate and the engine hesitates, the O2 shows low values around 0.03 and it won't rise until some time (in seconds) or until I lift my foot a little, then I feel some more power and the O2 says ~0.8-0.9... Low values mean a lean mixture?

 

I still have the data from my 15 minutes run if needed. I'm Using WinALDL.

 

Am I really dealing with a lean out problem? please!

 

Jerome

[GM-Tech]

Think of block learn/integrator numbers as fuel. 128 is ideal. Lower numbers mean less fuel. In other words, 114 is less than 128 so that means less fuel is being delivered because it's running rich. 144 is more fuel, and the ECM is adding fuel at that time because it's lean.

 

Block learn is long term fuel adjustments. Integrator is short term fuel adjustments.

[Jerther]

Thank you!

 

hmmm... most BLM numbers are low, so are the INT numbers. That would mean the ECU thinks the engine is generally running rich. So It gives it less fuel overall.

 

When it comes to acceleration, it doesnt give it enough fuel because it has learned not to give it much (BLT), unless it finally sees it is running lean and gives it some more (INT).

 

Does that sound like a consistent theory?

[Jerther]

What is the normal Idle speed? My engine idles around 925 RPM

[silverado3500]

That seems to be a bit high for idleing. It should be closer to 600-750

[Jerther]

Thank you! I'll check the IAC Valve.

[GM-Tech]

What is the normal Idle speed? My engine idles around 925 RPM

 

That's high. We've had some problems with those sucking in pieces of the throttle body gasket. That would give you a high idle AND a lean condition.

[Jerther]

Hmm, the gasket is new from last year (When I replaced the block)

 

Well, they're cheap, so i'll replace it.

[Jerther]

I have the IAC Valve im my hand right now. There is some black buildups in the spring area. The pintle is clean, but seems to have worn more seriously on one side, so it is not perfectly round.

 

heres a picture:

 

 

And found something interresting. I found the cause of the high Idle. There's something wrong with the throttle plate. There is a torx screw to ajust the minimum throttle position. I loosen the screw to see if it would change the idle speed. But it didn't. I had to push the lever back with my hand, and only then the idle dropped down. While doing this, I can hear and feel the throttle plates rubbing the metal.

 

I remember, the throttle used to get stuck, and that's why I fastened the screw...

 

Are the plates ajustable?

[GM-Tech]

And found something interresting. I found the cause of the high Idle.

 

Been so long that I had forgotten all about that problem. The hole in the throttle body for the throttle plate shaft gets wallowed out, and can cause either a sticking throttle or a vacuum leak, or both.

 

The only fix I know of is to replace the throttle body. Someone somewhere probably makes some sort of bushing kit for this problem.

[Jerther]

Not a problem, i've got a spare Throttle Body ;) And the gasket is on its way, getting it tomorrow and i'll do the swap tomorrow too. I've been thinking about doing that swap for a long time.

 

I'll get some break cleaner to clean the spare TB. This stuff does a really neat job

[Jerther]

Okay I did the swap. A few black smoke and everything went ok.

 

There was a sceal where the idle torx screw is so it should still be at factory setup. The engine idles around 750, but only when i'm not moving. Else, it idles around 1000.

 

And I think i'm getting more power. But you know, feelings...

 

I'm still at advanced timing.

 

This is the good news.

 

The hesitation is still present.

 

The spare throttle body has one more vacuum plug. I placed something to clog it. But what was supposed to be plugged there?

 

While the throttle body was off, I noticed some carbon inside the engine. A little bit on the passenger side, but more on the driver side. I know those engines are known to have buildups inside. If I can see some in the intake, I bet there are more at the valves and pistons.I'll have this cleaned before going any further.

 

What's the best way to clean the carbon inside the intake?

[GM-Tech]

The only way to clean the carbon out that would make any difference is to take it apart and dig out the big chunks and take it somewhere that can put it in a tank that will dissolve what's left.

[Jerther]

So much for Sea Foam...