05 Silverado 4.3 Dieseling

[Scott Cadwell]

My Silverado shortbed 4.3 has 95k miles and runs fine, idles well, has decent pickup, but when driving at 30-40 the engine loses a little power and makes what i'm calling a dieseling noise, almost like a pinging from bad gas "back in the day".....it's not a knocking sound, more like a rattle, but when I let off the accelerator just a little bit the noise stops and of course the truck slows down a little.....the noise also stops if I give it some gas, but them i'm going to fast in residential areas.......it also seems to get really crappy gas mileage for basically an empty truck, it's not loaded down, no trailer, just one passenger.......it almost seems like its in 4th gear and shouldn't be there yet??.....someone told me it might be the intake gasket, but its not leaking coolant anywhere inside or out......anybody have any ideas?

[Doug_Scott]

Your truck has knock sensors on it.  When they detect a sound in the engine casting(you can create a ping or knock due to timing/lower octane fuel than your manual specs) it will try programmed reactions to make it go away.  It will reduce timing first, then it will alter the fuel mapping to make it stop the pre-ignition.  If you have a scan tool that can display the amount of timing retard the PCM is setting.  The retarding timing will kill your economy.  Are you buying your gas at always the same station?  Its not unusual for today's fuels to become "stale" in a short period of time.  If you always use the same station, run your tank to near empty, and refill at the same brand fuel station, and put the same fuel you always use.  Run that tank to empty and see if there is any change.

 

Intake manifolds main job is to route air to the correct cylinder.  Managing the coolant is not the job to check on.  Check for vacuum leaks at each port where the intake runner meets the cylinder head.  You can use a propane bottle with a welding tip installed.  Open the valve, and don't light it.  With the engine running and cold, start it and with the propane tank and nozzle go around each port as best as you can.  If the engine changes sounds or speeds or both, that is pointing you to the correct cylinder.  Repeat this test on a fully warmed up engine the same way.

A vacuum leak can make the cylinder to run lean, causing cylinder temps to run hotter.  So much hotter it can even make parts inside the combustion chamber to start glowing red.  That glowing part will ignite the fuel way before the spark plug is made to fire.  That becomes pre-ignition or spark knock.

[Scott Cadwell]

I generally do run my tank to near empty before refills, but I don't use the same station and occasionally the truck will sit for a couple weeks without use in the Florida heat.....what if next time I refill with a higher octane or add some octane booster and see if the knocking goes away?.....Thanks for your help : )

[j-ten-ner]

Pull the spark plugs and look for unusual wear and tear or carbon deposits.

It may be an indicator which cylinder(s) is pre-igniting.

Pre-ignition can also be caused by carbon deposits on the piston or combustion chamber.

Does your engine use oil?

 

so long

j-ten-ner

[tlaw91]

Without driving your truck, it sounds to me like your torque converter clutch is applied too much for too long until you override it by accelerating. TCC apply starts at 30 mph under the correct conditions. So if let’s say your tcm commands 30% apply at 30 mph but the TCC solenoid valve has a clogged exhaust port, then it can’t dump fluid and ends up applying the converter clutch at 100%, which would give you a ‘always in 4th gear’ type feeling. It would also explain bad gas mileage and lack of power.

 

Anyways, just a guess!

 

-GM Tech

 

 

Sent from my iPhone using Tapatalk

 

[Doug_Scott]

18 hours ago, Scott Cadwell said:

I generally do run my tank to near empty before refills, but I don't use the same station and occasionally the truck will sit for a couple weeks without use in the Florida heat.....what if next time I refill with a higher octane or add some octane booster and see if the knocking goes away?.....Thanks for your help : )

The post from tlaw91 regarding TCC acting up brings up a good point, and should be real easy to verify.  When it starts acting up, just touch the brake pedal with your left foot, only press hard enough to turn the brake lights on.  Pressing the brake pedal will unlock the torque converter.  Keeping your foot on the gas pedal will show you pretty much right away if that is the issue.  The engine should increase in speed, without having to downshift.  The trick is to just leave your right foot where it is on the gas pedal, and only press the brake pedal a very small amount.

[tlaw91]

You can also put it in manual 3


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[Doug_Scott]

3 hours ago, tlaw91 said:

You can also put it in manual 3


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Yes you can, but, you are effectively making 2 "gear" changes.  I was brought up to limit changes to just one at a time when troubleshooting.  Besides, touching the brake pedal does not require you to look at the shift quadrant while downshifting. 

 

 

[Scott Cadwell]

I will try the brake pedal thing tomorrow and post results, Thanks!

[Scott Cadwell]

Well I tried downshifting into 3rd gear from drive at the speed when it was making the "dieseling" noise and it went away, however I discovered that it's making that noise in every gear once it warms up until I give it more gas, so i'm thinking now it may be that low octane problem?.....or maybe it needs new plugs?.......I bought it from the original owner a few months ago but I don't think he took care of it, I will pull some plugs when I have time and report back........it's not a loud noise but you can tell something is off, but when you give it gas it goes away, not hard enough to downshift just a little extra gas....Weird

[Scott Cadwell]

I couldn't force myself to hit the brake while my right foot was on the gas, it's like something that was drilled into my head 40+ years ago......like not pointing a gun at someone, loaded or not!!

[Scott Cadwell]

Well I finally emptied the gas tank and refilled with 93 octane, drove it 30 miles on the highway and it ran fine like normal for high speed, but once I got off the interstate it does the same thing.....so its not bad gas or low octane......I bet it has something to do with what Doug Scott mentioned in the first reply, I don't have a scan tool or propane bottle so i'll have to find a mechanic that does, hopefully they don't get all pissy when I try to tell them what I think is wrong!!.......so could a vacuum leaking intake gasket cause these symptoms?

[Doug_Scott]

On 2/24/2020 at 6:57 PM, Scott Cadwell said:

I couldn't force myself to hit the brake while my right foot was on the gas, it's like something that was drilled into my head 40+ years ago......like not pointing a gun at someone, loaded or not!!

To be clear here, you do not need to "hit the brakes", you just need to depress the pedal far enough to make the brake lights come on. It will be about a half inch of pedal travel. 

Can you record this happening and post it to you tube? 

 

[Doug_Scott]

Forgot to mention, if your truck has cruise control, set it for the speed that your issue appears at, does it do the same thing?  If you just touch the brake pedal and apply a small amount of pressure you will find out exactly how far you have to press the brake with your foot still on the gas.  Cruise control shuts off around the time the brake lights come on.  You still need to have your foot on the gas to test the lockup converter, testing with cruise control is just to show you how hard you need to apply the brakes.  If your foot is off the gas, that negates the test.

[Scott Cadwell]

My stepson was visiting today, he's a mechanic in Maine and we took it for a ride. He is convinced the noise is just a rattling heat shield and/or possibly the CAT degrading inside.