1999 extended cab Silverado 4.8/4l60e

[Ethan.jones01]

So I recently purchased a 1999 Silverado extended cab with the 4.8/4l60e. The truck runs and drives good for the most part and sounds healthy but it’s got some minor issues, one being that when in Drive the Rpms will not exceed 2500rpms even if I put the pedal to the floor. In neutral the engine will rev up 4-5k rpms but while in Drive it will not. Does anyone have any insight on this issue? Could it be a transmission issue or something else? No CEL and truck runs pretty good on the highway no issues accelerating gradually/slowly and holds highway speeds of 60+ easily

 

[stutaeng]

Umm, that's kinda strange. I'm assuming you didn't realize this when you test drove it before you bought it? They didn't tell you anything about that being an issue?

Can you feel/hear the transmission shifting through the 4 gears?

Do you have a scanner that has live data so you can monitor things like Throttle Position Sensor to see if the ECU is recognizing where the pedal position is?

 

[ORVietVet]

Welcome to the forum from Oregon. You are gonna want to go to the #1 post and at top right there are 3 dots and click on that and edit your post title to read 4.8 instead of 3.8.

Sounds like you have the start of catalytic converter(s) clogging up creating to much back pressure against exhaust flow. OR, you could have a problem with fuel pressure. To check the fuel pressure attach a fuel pressure gauge at the fuel rail on top of engine. Gas engine fuel pressure should be 55 to 62 psi and flex fuel should be 48 to 54 psi. Also, you could have a clogged fuel filter. Not enough to shut off the full flow but enough to restrict the flow and cause power problems. If you have no idea of the age of the fuel filter, change it and see what happens but still do the fuel pressure test, especially before the filter change and then after. To check for back pressure problems, you need to run a back pressure test on the exhaust system. Sometimes you can have a CEL with back pressure problems and sometimes not.

Also recommend doing what @stutaeng recommended. Live data scanners are not that expensive.

 

[Ethan.jones01]

Umm, that's kinda strange. I'm assuming you didn't realize this when you test drove it before you bought it? They didn't tell you anything about that being an issue?

Can you feel/hear the transmission shifting through the 4 gears?

Do you have a scanner that has live data so you can monitor things like Throttle Position Sensor to see if the ECU is recognizing where the pedal position is?

Umm, that's kinda strange. I'm assuming you didn't realize this when you test drove it before you bought it? They didn't tell you anything about that being an issue?

Can you feel/hear the transmission shifting through the 4 gears?

Do you have a scanner that has live data so you can monitor things like Throttle Position Sensor to see if the ECU is recognizing where the pedal position is?

i test drove it but no I didn’t really notice the issue mainly because I didn’t floor it or do any hard acceleration. It’s not a perfect truck I mean I paid 4K cash for it and it’s 25ish years old. I expect issues. Transmission seems to be shifting fine however guy I got it from told me the carrier bearing is going out. Loud whirring sound when like 20+mph but I’m starting to think torque converter. I don’t have a live data scanner but I was contemplating replacing the TPS anyway.

 

[Ethan.jones01]

Welcome to the forum from Oregon. You are gonna want to go to the #1 post and at top right there are 3 dots and click on that and edit your post title to read 4.8 instead of 3.8.

Sounds like you have the start of catalytic converter(s) clogging up creating to much back pressure against exhaust flow. OR, you could have a problem with fuel pressure. To check the fuel pressure attach a fuel pressure gauge at the fuel rail on top of engine. Gas engine fuel pressure should be 55 to 62 psi and flex fuel should be 48 to 54 psi. Also, you could have a clogged fuel filter. Not enough to shut off the full flow but enough to restrict the flow and cause power problems. If you have no idea of the age of the fuel filter, change it and see what happens but still do the fuel pressure test, especially before the filter change and then after. To check for back pressure problems, you need to run a back pressure test on the exhaust system. Sometimes you can have a CEL with back pressure problems and sometimes not.

Also recommend doing what @stutaeng recommended. Live data scanners are not that expensive.

Guy I bought it from said it’s got a new catalytic converter. I’ll check fuel pressure and probably change the fuel filter anyhow. I just bought the truck last Friday and I plan on doing a full tune up and replace a lot of things anyways but just trying to rule this problem out.

 

[ORVietVet]

Are you sure it has a carrier bearing. Thar would need to be a very long rear driveshaft. The noise is only at 20 mph? When accelerating? Coasting?

 

[Ethan.jones01]

Are you sure it has a carrier bearing. Thar would need to be a very long rear driveshaft. The noise is only at 20 mph? When accelerating? Coasting?

It’s only under load when accelerating basically and yes it’s a two piece drive shaft. Only start hearin it above 20-30mph but if i let off throttle and coast at any speeds it disappears. That’s why I’m leaning more towards it being the torque converter. Along with the acceleration issue

 

[stutaeng]

i test drove it but no I didn’t really notice the issue mainly because I didn’t floor it or do any hard acceleration. It’s not a perfect truck I mean I paid 4K cash for it and it’s 25ish years old. I expect issues. Transmission seems to be shifting fine however guy I got it from told me the carrier bearing is going out. Loud whirring sound when like 20+mph but I’m starting to think torque converter. I don’t have a live data scanner but I was contemplating replacing the TPS anyway.

Understand.

Carrier bearing? Ok, yeah that's likely. It's a 2wd or 4x4? My 99 ECSB 2wd has a carrier bearing. I replaced it at 260k and it was pretty warn out.

TPS? I wouldn't replace sensors "just because 'surely, it cain't hurt' at this mileage." Aftermarket sensors are hit/miss, then you don't know what's causing what. We see this too often on truck forums.

Learn and invest diagnostics/tools, or that it to a GOOD, trustworthy mechanic.

 

[Ethan.jones01]

Understand.

Carrier bearing? Ok, yeah that's likely. It's a 2wd or 4x4? My 99 ECSB 2wd has a carrier bearing. I replaced it at 260k and it was pretty warn out.

TPS? I wouldn't replace sensors "just because 'surely, it cain't hurt' at this mileage." Aftermarket sensors are hit/miss, then you don't know what's causing what. We see this too often on truck forums.

Learn and invest diagnostics/tools, or that it to a GOOD, trustworthy mechanic.

It’s 2wd. Body has 257k on it but motor has been replaced and only has about 130k on it but I was thinking carrier bearing but wouldn’t it still make noise if I was coasting at 50mph? If I let off throttle it’ll dissapear and the second I press the pedal again it comes back

 

[ORVietVet]

I would certainly lean toward the carrier bearing. You were told it was needed and they tend to make more noise under a load. Check the rear diff fluid level and for debris in the fluid. Might be time to change anyway. If I buy ANY used vehicle and have no records I can count on for fluid change intervals, I change them all. No ifs ands or buts. Check the carrier bearing while under there. 2wd long bed and extended cab or crew cab? Tend not to see a carrier bearing on a 4wd, even a long bed, because the transfer case shortens the driveshaft.

If you can record the noise and post it here, do it. You will have to upload the video to a place like you tube and then post the link here.

 

[stutaeng]

It’s 2wd. Body has 257k on it but motor has been replaced and only has about 130k on it but I was thinking carrier bearing but wouldn’t it still make noise if I was coasting at 50mph? If I let off throttle it’ll dissapear and the second I press the pedal again it comes back

Sounds like a rear end issue, like the drive side of the ring and pinion gear. Maybe you should pop the differential cover and do an inspection. Probably should replace the fluid anyway as @ORVietVet is suggesting. Definitely if you don't have service records from the previous owner...

(Even though my carrier bearing was pretty worn on my truck, it didn't make any noises, so IDK...)

 

[Ethan.jones01]

Welcome to the forum from Oregon. You are gonna want to go to the #1 post and at top right there are 3 dots and click on that and edit your post title to read 4.8 instead of 3.8.

Sounds like you have the start of catalytic converter(s) clogging up creating to much back pressure against exhaust flow. OR, you could have a problem with fuel pressure. To check the fuel pressure attach a fuel pressure gauge at the fuel rail on top of engine. Gas engine fuel pressure should be 55 to 62 psi and flex fuel should be 48 to 54 psi. Also, you could have a clogged fuel filter. Not enough to shut off the full flow but enough to restrict the flow and cause power problems. If you have no idea of the age of the fuel filter, change it and see what happens but still do the fuel pressure test, especially before the filter change and then after. To check for back pressure problems, you need to run a back pressure test on the exhaust system. Sometimes you can have a CEL with back pressure problems and sometimes not.

Also recommend doing what @stutaeng recommended. Live data scanners are not that expensive.

So another question. I’ve noticed my oil pressure gauge never really moves. It’s pretty much always at 60. When I kill the engine it’ll sit at 40 with no power. Under throttle and it’ll move a little bit but no really noticeable changes. Is this normal? What should typical readings be? I plan on changing Oil and rear diff fluid tomorrow and filters (Oil,fuel,Air). Probably going to grab tester gauges for oil and fuel pressure as well. 1st pic is otw to work this evening after warmed up, 2nd is “cold” start after sitting for 6hrs

 

[INW-Iron-Steel]

So another question. I’ve noticed my oil pressure gauge never really moves. It’s pretty much always at 60. When I kill the engine it’ll sit at 40 with no power. Under throttle and it’ll move a little bit but no really noticeable changes. Is this normal? What should typical readings be? I plan on changing Oil and rear diff fluid tomorrow and filters (Oil,fuel,Air). Probably going to grab tester gauges for oil and fuel pressure as well. 1st pic is otw to work this evening after warmed up, 2nd is “cold” start after sitting for 6hrs

Because of this, I would assume your stepper motor is on the way out. This is a very common issue with GMT800 clusters. When one stepper goes, it is common for the others to follow. I replaced my cluster at 230-ish because of a failed oil pressure stepper motor. Mine would move to the right when I started then engine and then get stuck. When I re-started the engine the next time it would move again and then get stuck at a higher pressure until the needle was stuck maxed out all the way to the right.

IMO, The fuel pressure gauge is a good idea, but a decent scan tool will be able to tell you your oil pressure as seen by the ECM. This would be a good way to verify gauge/cluster issues VS oil pressure sender issues (these can also be somewhat troublesome on LS based engines). When you have the diff cover off, jack the truck up and put it on jack-stands. Pull the rear brakes (this eliminates drag on the axles) and drop the rear of the driveshaft. Spin the yoke by hand and feel how the bearing is moving. Try to move the pinion up and down at the yoke. Also try to move it in and out. You are looking for play here and don't want any. This is different from backlash, which you will feel/hear as a bang-bang as you turn the yoke back and forth before the gears engage. If you have some grease, it works OK as gear compound and you could run a pattern on the gears.

FWIW, the 99 4.8 in my Sierra holds 30PSI oil pressure at hot idle. I see just over 40PSI cruising at temp on the freeway in overdrive. That is with my preferred 10w30 HDEO. Somewhat recently I was seeing 20PSI at hot idle, but that was a one-off type of situation.

 

[ORVietVet]

Like @INW-Iron-Steel has said about the oil pressure stepper motor, I bet it is bad. I bought my listed truck on Dec. 1 and all was good. Then it started to act like the 40 psi level was actually the "0" pressure level. Everything started at 40 psi and worked from there. It is not unusual for the gauge to shut off at around 30 psi to 40 psi when you shut engine off. When you then do the KOEO, it should drop close to zero and then of course show actual pressure when start the engine. Is also likely that the other stepper motors will start to malfunction. You can use a live data scanner and measure voltage that will equate to oil pressure. I bet yours is off. I would contact Digital Dash Solutions and look at the site and you can customize the look if you want and even add a transmission temp gauge that is just plug and play. I am going to supply a link for them and an oil pressure voltage pic to compare with.

Silverado Cluster - Replacement Sales

DIGITAL-DASH-SOLUTIONS SPECIALIZING IN NEW & FACTORY REBUILT REPLACEMENT CLUSTERS SALES FOR 2003 2004 2005 2006 CHEVY SILVERADO, TAHOE, SUBURBAN & AVALANCHE - GMC SIERRA & YUKON - CADILLAC ESCALADE & H2 HUMMER.

www.fixmygauges.com

 

[stutaeng]

99 has the air core motors, not stepper motors. Not sure if that matters. I want to say air core motors are not "too" common to have that issue, but I guess anything can fail at this point? IDK

 

[ORVietVet]

99 has the air core motors, not stepper motors. Not sure if that matters. I want to say air core motors are not "too" common to have that issue, but I guess anything can fail at this point? IDK

Did not know that. Now I have to research for what an air core motor is and how they work.

Thanks a freakin' heep for making my day more difficult. Sheeesh! LOL!

 

[ORVietVet]

So, there is an internal fluid that breaks down and starts the problems with the gauge readings. The 1999 is considered a GMT800 but they used air core motors into 1999. I guess Digital dash Solutions can tell you when you contact them. Thanks for the info @stutaeng.

 

[stutaeng]

So, there is an internal fluid that breaks down and starts the problems with the gauge readings. The 1999 is considered a GMT800 but they used air core motors into 1999. I guess Digital dash Solutions can tell you when you contact them. Thanks for the info @stutaeng.

Cool. Yeah, so I think the air core motors is a 99-02 thing, with stepper motors being a 03-07 classic?

I mean, they all look the same or similar (the instrument clusters), but the motors I guess are just different.

 

[INW-Iron-Steel]

99 has the air core motors, not stepper motors. Not sure if that matters. I want to say air core motors are not "too" common to have that issue, but I guess anything can fail at this point? IDK

Cool. Yeah, so I think the air core motors is a 99-02 thing, with stepper motors being a 03-07 classic?

I mean, they all look the same or similar (the instrument clusters), but the motors I guess are just different.

Man, thanks for the education! I had no idea they were different! Admittedly, I have worked on very few early gmt800s.
I think the clusters may be a little different. 99-02 looks to have the information readout over on the left, while newer clusters moved this down underneath the gear selection indicator. Blue gauges are the 99-02 cluster. I wonder if these are interchangeable between all years?

 

[DateNight]

you can only

Man, thanks for the education! I had no idea they were different! Admittedly, I have worked on very few early gmt800s.
I think the clusters may be a little different. 99-02 looks to have the information readout over on the left, while newer clusters moved this down underneath the gear selection indicator. Blue gauges are the 99-02 cluster. I wonder if these are interchangeable between all years?
View attachment 2359View attachment 2360

99-02 will only work in 99-02 trucks and full size SUV, same goes for the 03-07 gauges.

But, 99-02 gauges that have trans temp(Denali, Escalade and hd trucks) will work in a truck that originally did not have a trans temp cluster.