Missing only when going up a hill

[Dinos]

I have a 1998 C1500 Silverado with a 350 and has 104,000 miles "Original Owner" The truck starts instantly, idles very smooth and quite, takes off with lots of power and torque, can drive in the city or on the freeway no issues at all, I can run it on the freeway 75-80 with zero issues, I can pull a long gradual grade at 70-80 MPH no issues. However the truck will start missing and shaking only if I am pulling a steeper hill and only driving around 50 MPH, once I reach the top of the hill and back off the throttle within 10~15 seconds it goes back to smooth and powerful, most of the time it does NOT throw a code unless I really force it up the hill and not backing off the throttle and will then throw the P0300 "random misfire" and nothing else.  

 

What I have done so far:

 

Rotor, Cap, Wires, Plugs, Fuel Filter, Cleaned throttle body, Replaced TPS, Replaced the EGR valve, replaced the fuel Pump 2 years ago, It just passed the Calif. emissions test with no issues, Tested the fuel pressure, with key on 63 PSI, engine Idling 58 PSI, when snapping the throttle open jumps back to 63 PSI, I did monitor the fuel pressure during the smog test and under a 50% load pressure averaged 60~62 PSI. I did a fuel pressure leak down test and it held at 62 for over 10 Min. I did a vacuum test at an Idle it pulls around 18~20 steady, Cleaned the MAF sensor. I am a bit stumped at this point mainly because the truck runs flawless other than pulling a hill at 50 MPH, I have read many articles about this issue with no real definitive answers and was hoping this group could lead me to the answer. I feel like I am missing something simple..

 

Thanks!

Dinos...

   

 

[Ian Mills]

I wonder if this is a heat related issue. Do you notice this happening more in the summer or on hot days?

[Colossus]

At 50 MPH your tranny and torque converter is going to be at it's weakest point for 4th gear.  I've experienced shudder and such when towing on older trucks at that range and the truck shifts into 4th.  bumping it down to 3rd until i can get the speed up helped in that situation. 

[Dinos]

1 hour ago, Ian Mills said:

I wonder if this is a heat related issue. Do you notice this happening more in the summer or on hot days?

I don't recall it doing it any less in the cooler weather.

[Dinos]

1 hour ago, Colossus said:

At 50 MPH your tranny and torque converter is going to be at it's weakest point for 4th gear.  I've experienced shudder and such when towing on older trucks at that range and the truck shifts into 4th.  bumping it down to 3rd until i can get the speed up helped in that situation. 

What I have tried while running up the hill was to lightly press the brake pedal just enough to activate the brake lights thus disengaging the torque converter lock and it still did it as well, I don't remember if I manually kicked it out of overdrive and into 3rd before going the hill, I will try that in the morning. 

 

Are your thoughts while the Trans is shuttering / pulsing creating erratic load the ECM module is trying compensate thus throwing it off, what I did forget to mention is once the shaking / missing starts if I put the Trans. into neutral and rev up the engine the engine is missing really bad for about 10 Secs then smoothens out back to normal.

[chem_man]

Have you checked your ignition coil?  Perhaps under that load, it might be giving out.

[Dinos]

No I haven't replaced that yet..

Thanks!

Dino's

 

[Dinos]

On 9/27/2020 at 11:36 AM, Colossus said:

At 50 MPH your tranny and torque converter is going to be at it's weakest point for 4th gear.  I've experienced shudder and such when towing on older trucks at that range and the truck shifts into 4th.  bumping it down to 3rd until i can get the speed up helped in that situation. 

I tried running up the hill with the Trans in 3rd and it still starting missing about 2/3 of the way up the hill.

 

Thanks!

Dino's

 

[Mike GMC]

Cats beginning to clog?  Can you replicate on flat ground with a full throttle pull?  Or maybe several in a row?  Otherwise, I would agree, coils, wires and plugs would be my next step.

Edited September 29, 2020 by Mike GMC

[Dinos]

While getting on the freeway today from about 15 MPH  I hammered the throttle And didn't let up until about 85 MPH, and it did seem to start to have a slight miss but no were as bad as it does going up a hill, of course a hill is producing a higher load, but I didn't get a check engine light....

 

Thanks!

Dino...

[riverbanks]

Following many forums,it keeps me sharp,on another forum,not to long ago,somewhat similar problem,guy changed the pcm,fixed,he was out of ideas also,the reply from the forum was,"wow,wouldn't have guessed that"

[Colossus]

On 9/28/2020 at 5:26 PM, Dinos said:

I tried running up the hill with the Trans in 3rd and it still starting missing about 2/3 of the way up the hill.

 

Thanks!

Dino's

 

well thanks for trying.  where are the RPMs again when this is happening? 

[Brayden1999]

I am having a similar problem with my 2004 Chevy Tahoe. No miss at all driving except when I am at highway cruising speeds going up hill. About half way up the hill the check engine light will start to flash but if I let off the gas for a second and then get back into it it goes back to normal. Any ideas? Let me know if I can provide anymore info. Thanks!