I've just bought a old 1998 Jimmy with a 4.3L vortec engine. I'm in Quebec Canada in now in the winter season.

It start very well, have a perfect iddle with no fluctuation at about 800 rmp. Don't have any lost of power when stomp on the gaz. Here is my problem. On the cold morning It have some kind of pinging or shudder (like jumping RPM or misfire) at all RPM between 1200 ant 2400 RPM when going off the street at all shifting speed. This is slowing down when the engine is getting hotter and even stop when engine had run for more than 30 min. When The engine is hot and rolling the street at 60 mph, the pinging or shudder came back at 2200-2400 RPM and seem to be like constant and
In the vehicule it seems to feel like a missing explosion in the sequence. and this is like very regular timing.

Strange thing, If the vehicule get into a heated garage for 1 hour and then go for a test ride, no pinging or shudder at any RPM.

The EGR is new and clean, the spark plug wire had been changes for NGK wires (blue) by the previous owner, the rotor cap was loose on 1 side and I screw it back in place but didn't check the rotor. I had injector cleaner 2 times, and seafoam in the intake.

I planned a spark plug change for platnium type and if this did not solve the problem, I will change the fuel filter. I did't check the cat, there is no check engine or sulfur smell on the exaust that can link to it.

I don't know much about MAF, IAS or TPS or knock sensor can cause the problem because it is not the same on cold, wet and on dry hot engine.

Please help, Any advice welcome because I want to keep the truck and use it everyday and to be able to have my project done.

 

Start with changing the rotor and cap. Very sensitive area. NOT CHEAP ONES!!! Only AC Delco!!! If AC Delco not available, Echlin or Balkamp. Not Standard or Wells

While that cap is off check small vent areas at bottom of dist around the sensors, spray out with intake cleaner or electronic cleaner, not just any spray solvent.
Note: this is not just a list of troubleshooting steps. Do all this whether or not the cap and rotor fix the problem.

I would suggest your vehicle has pending codes.
Find an auto parts store that will read your engine codes. Write the actual codes down - DONT BOTHER with anyone's definitions of them- and report them here.

Check the MAF: When in situation causing the problem disconnect the MAF connector, drive that way and see if symptoms are the same. May hesitate some but should run smoother if MAF is part of problem.

Clean the MAF anyway. But it's good to know the cause
Here's the definitive instructions on doing it right. I've been reading MAF cleaning how-tos since about '95 and everything on there is 100% right. including that screen which is not there to keep out bugs!
https://blazerforum.com/forum/tech-a...leaning-40420/

Spark Plugs: AcDelco or NGK only. NOT Bosch or Champion

 

I'm gonna replace the rotor and rotor cap and keep you posted.

I had bouth the champion platinum spark plug, I should get them back and buy NGK instead

For MAF, I'll try that this weekend also.

Distributor rotor and cap had been changed... nothing happened keep pinging, cleaned MAF, nothing change, clean trottle body, helped a bit but not had solve anything. tried to run MAF disconnected, seem to had stop but engine is hot so neet to wait tomorrow to see on cold start.

Did you thing I need to replace fuel filter and or TPS? I didn't notice any lost of power at higher rev.

will also add 6 new NGK platinum spark plug this week-end.

This is driving me crazy

 

No change on plugged or unplugged MAF this morning, still shudder on cold engine.

I've bought a USB obd2 code reader and software, let see what it's gonna give us. I'll be able to check this:

■Read diagnostic trouble codes, both generic and manufacturer-specific, and display their meaning (over 3000 generic code definitions in the database).
■ Clear trouble codes and turn off the MIL ("Check Engine" light)
■ Display current sensor data, including:
■ Engine RPM
■ Calculated Load Value
■ Coolant Temperature
■ Fuel System Status
■ Vehicle Speed
■ Short Term Fuel Trim
■ Long Term Fuel Trim
■ Intake Manifold Pressure
■ Timing Advance
■ Intake Air Temperature
■ Air Flow Rate
■ Absolute Throttle Position
■ Oxygen sensor voltages/associated short term fuel trims
■ Fuel System status
■ Fuel Pressure
■ Many others...

With a laptop connected in vehicule, I'll be able to scan in real time the shudder or I'm wrong?

 

Let me recap: I made a mistake my Jimmy is a 1997 and not a 1998, my mistake


Now is more update:

It start very well and easy, have a nice iddle with no fluctuation at about 1000 rmp. It have some kind of pinging or shudder hesitation or kiccup at all RPM between 1000 ant 2500 RPM when going off the street at all shifting speed. I have this issue at any engine temperature. Since my first post, it seems deteriotating even with the parts changed. When The engine is hot and rolling the street at 60 mph, the hesitation keep going at any transmission gear. In the vehicule it seems to feel like a missing explosion in the sequence and this is like a heart beat in the explosion.

The EGR is clean, the spark plug wire had been changes for NGK wires (blue) by the previous owner I didn't check them because I have no idea how to, the rotor cap was loose on 1 side and I screw it back in place but I've decide to change it but the only part I was able to have was a Standard brand but it changes nothing on the problem I had injector cleaner 2 times, and seafoam in the intake. I clean the trottle body 3 times and twice the MAF with carb cleaner. I've changes the spark plugs or NGK platinum.

Today, it was worse and I tought running on 5 cylinders instead of 6 just to let you imagine how it was hesitating and had a rough idle. I've decide to bring it to a garage and they will check it tomorrow.

My worst nightmare is if the problem is the fuel pump because it worth 300$ and i've paid the truck 1500$ My only concern is: Why the truck is starting perfect without any hesitation and don't lost power on full trottle it it is the fuel pump?

A clogged fuel filter or cat would had no change in working quality regarding the outside temperature?

Does the injector spyder is a common issue on a truck of 140k miles?

Is there many vacuum lines on those engine?

And for my ending, the truck is gazoholic... it run at 10-12 miles per gallons now...

I'll keep you posted on my problem

 

If you were running on 5 cylinders instead of 6, you should have the check engine light on. That means it's time to read the codes.

At 10-12 MPG, you should watch the "fuel systems status" for "Open Loop" and "Closed Loop". If you see "Closed Loop" when it warms up, you might want to look for a leaky spyder assembly.

 

After pulling the codes, first thing is to find out why you are idling at 1000 rpm. These engines idle at 650
High idle points often to vacuum leak, lean conditions cause ping.

If your PCM and knock sensor are running right that will cause first a spark retard then a richer mixture.

 

Thanks, I'll have it scan and tested tomorrow. Sunday, i also had issue with getting on a stop but engine wasn't slowing RPM and was staying at 1500 rpm, i had to race it then it got down to 1000 rpm... maybe a vacuum problem is logical.

I'll keep you posted

 

OK here is the scan status... misfire on cylinder 3 and 4 and check for spark problem... between 1400 and 2200 rpm, the engine was throtteling on 4 or 5 cylinder and not burning the fuel. Finaly, put a recycled distributor shaft in it and payed 400$ box for the part and reaserch to solve my problem.

Truck is running good now. Should put this possibility of bad parts in people that having hesitation, shudder, hiccup or any other problem like mine.