GMT400 - a love/hate relationship

[jonathon]

I thought I’d share my GMT400 adventure. These trucks are plentiful and cheap right now, but that won’t always be the case.

Back in the beginning of 2018 it was apparent I needed a truck. We could only afford one newer vehicle so I decided to go for an early GMT400, preferably a K2500 or K3500. We had a 2017 T4R that was later sold so we could buy an Outback. I found a light duty K2500 extended cab on Craigslist and it looked straight for 210k miles. It was advertised for $3200 and had a Goodwrench L05 crate engine put in at 100k miles in 1998 and the 700r4. There was paper to back up the engine and transmission. Looking at the truck it was apparent there was a lot of deferred maintenance and poor quality repair work. The seller had replaced the starter and took out the knock sensor wire in the process. This caused a code 43 and the engine to run in limp mode. The thermal linear actuator was always engaged so he just disconnected it. The entire front end was toast too. The plus side was otherwise unmolested and the truck was clean with an intact, albeit tired, interior. It had a set of black steelies with old Michelin LTX 31’s on it. I offered $2000 and after much deliberation he took it. I still think I got a good deal.


The day I brought it home.


First obvious issue was a major vacuum leak from the evap lines. Fixed with new vacuum line.


Door roller and hinge pins next. Typical GMT400 problems.


The source of much grief. Turned out the previous owner snapped the knock sensor wire when pulling the starter. He found a broken wire and repaired it. Wrong wire! I had to run a new wire from the electronic spark control module to the sensor.


These diagrams really helped me with the code 43 and knock sensor.

I did a full tune up with plugs, wires, cap, and rotor. I also replaced the coil as it was arcing bad. AC Delco parts are a must on a TBI engine or you will eventually have issues. My EGR valve was not so great either so I replaced it with a new AC Delco part.


Then I just drove it. We went camping, day trips, forest road adventures, and did work. Long bed extra cab is just about perfect.

To be continued...

 

[jonathon]

This winter we made several trips into the forest to go sledding. On one of those trips I thought my starter solenoid was going out as the starter would spin but not engage the flywheel. Then it just stopped all together.


I found my starter hanging by one loose bolt and the dust cover broken. Got dragged home by a neighbor. Ordered two new AC Delco starter bolts and torqued them to spec. Thankfully the flywheel wasn’t damaged. I have a new dust cover but I need to pull the y pipe to install it. Previous owners suck!!

I then replaced the three u joints and center support bearing. The joints and CSB were original to the truck and toasted.

Decided to tackle the front end next. Truck wandered bad and clanked over bumps. Lots of play in the steering too.


Pulled my steering gear to find it seriously bubba’d and the output leaking bad. I ended up with a new unit from Red Head Steering Gears. New TRE’s, idler arm, pitman arm, and Bilstein 4600 shocks and I buttoned it up for an alignment. My mechanic advised my balljoints were bad too so...


Back apart she went. Was a pretty easy job. Got it aligned and it drives like new.


Did some more truck stuff around the house.


A day trip to Badger Lake in the Mt Hood national forest via the 3550 road. This road is pretty much 4x4 only and she really got to do some work. No other pics, but the trip went without a hitch.

We had a trip planned to Cottonwood Canyon on the John Day River. During my pre trip inspection I found a nail in the tread of the front driver side tire. Since we were still in town I dropped by my tire shop to get it patched. The bead disintegrated due to dry rot. They hooked me up with a used 31 to get me on the road and didn’t even charge me. I ended up ordering a new set of 31’s. I wanted 33’s but the TBI 5.7 would definitely need a gear upgrade so I stayed with 31x10.50r15 tires.



On the road to Cottonwood. Continuing...

 

[jonathon]

Cottonwood Canyon. We have a 10x7 Springbar Campsite. Easily the best tent I’ve ever slept in. Comfortable, quiet, and no issues with wind. It is super windy on the John Day.



Got back and had the new tires installed. Blacklion Voracio 31x10.50r15’s. They were cheap, C rated, 16/32” worth of AT tread, and severe weather rated. They will do until I get the gear or motor for 33’s.

Where am I now? I still have quite the to do list:

• Intake gasket
• Exhaust manifolds, cat, and y pipe.
• Reseal heater box for winter

I had an oil sample done and unfortunately there is potassium and sodium in the oil, meaning a coolant leak. No milk shake, overheating, or white smoke so I’m hopeful it’s the intake and not a head gasket. Intake is cheap and easy so I will be doing that soon. If it’s a head gasket? I may do a 5.3/4L60E swap.

 

[CodyY]

Its probably intake gaskets, they cause a funky vacuum leak too. Be sure to use plenty thick of a bead on the rear sealing surface. I had to do the intake gaskets 3x once because of a oil drip.

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[jonathon]

Its probably intake gaskets, they cause a funky vacuum leak too. Be sure to use plenty thick of a bead on the rear sealing surface. I had to do the intake gaskets 3x once because of a oil drip.

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Did you buy new intake bolts?

I’m planning on tackling this in mid September. Also have a throttle body rebuild kit. I haven’t had to add coolant since I tightened a loose hose clamp 1200 miles ago and the truck is running great.

 

[vargsmetal]

Great truck and great thread! Since you have the LD2500 you should have the same 3.73 gears (rpo code GT4) as my F44 1500. I've been running 33s on mine and it did fine with the 350/700R4 even when towing. 4.10 or 4.56 would be better but I don't know how easy it would be to get them for the 8.5 front diff, or if it's even worth while. Just like putting 4.10 or 4.56 in a 10 bolt rear, the pinion starts getting tiny.

The 5.3/4L80e I have in mine eeked out 18mpg on the last tank, finally equaling the 350/700R4 I had before. The LS is a great swap for these trucks.

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[Flazer]

Following. I have a 2000 K2500HD I picked up as a government surplus vehicle. Been looking into mods and maintenance for it. 120k miles and going strong.

Only thing weird is it's an 8 lug semi-float 14 bolt. Usually they're 6 lug when they have the semi-float.

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[CodyY]

Did you buy new intake bolts?

I’m planning on tackling this in mid September. Also have a throttle body rebuild kit. I haven’t had to add coolant since I tightened a loose hose clamp 1200 miles ago and the truck is running great.

Never found a need to replace intake bolts unless you're just getting fancy, or starting from scratch.

I'd probably be tempted to leave it alone, but the intake gaskets aren't too bad to replace. Gives you a good chance to clean up a lot of dirty stuff on the motor.

I dont like fighting the rubber end pieces that come with the gaskets, so I use straight Grey FIPG. Easy when a motor is on the stand to make sure you sealed it good, but instead of the usual ³/¹⁶ bead, by the time it was all said and done, I wound up with a ⅜ bead and let it sit overnight before starting.

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[jonathon]

Decided to tackle the intake today, or at least take it off. No issues other than a snapped bolt - front most driver side bolt. I am not surprised as this where it was leaking.


Got chased inside by the weather but the lifter valley and intake were both very clean. Only had a chance to snap a photo of the intake.




Also figured out why my heat sucks. I intentionally broke the heater quick connect off to remove it with an extractor and replace with a nipple. It was packed with what I’m assuming is stop leak. I do recall now that the previous owner tried to fix a leaking water pump with stop leak



The coolant passages I can see otherwise look clear.

I need to source at least one new intake stud.

 

[CodyY]

Friggin shadetree mechanics and their miracles in a bottle.

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[vargsmetal]

Following. I have a 2000 K2500HD I picked up as a government surplus vehicle. Been looking into mods and maintenance for it. 120k miles and going strong.

Only thing weird is it's an 8 lug semi-float 14 bolt. Usually they're 6 lug when they have the semi-float.

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Nice truck! The 8 lug 14BSF is actually more common than the 6 lug version, and was the only way it came prior to 88.

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[jonathon]

I was going to slap the intake on today but experienced some difficulties caused by me not removing the the oil pressure switch. Will get it tomorrow.

I did fix my loose distributor cap. One of the holes in the distributor was stripped. Stock size appears to be 6-32. Drilled out with a #29 bit and tapped to 8-32. Saved me from replacing my distributor.

 

[lilkia]

Following. I have a 2000 K2500HD I picked up as a government surplus vehicle. Been looking into mods and maintenance for it. 120k miles and going strong.

Only thing weird is it's an 8 lug semi-float 14 bolt. Usually they're 6 lug when they have the semi-float.

Sent from my PH-1 using Tapatalk

Nice truck! The 8 lug 14BSF is actually more common than the 6 lug version, and was the only way it came prior to 88.

Sent from my SM-G965U using Tapatalk

Its a crapshoot on the gmt400 2500s. There were 7200lb and 8600lb gvw trucks. I have never seen the 7200 with anything but the SF axle. I have seen the 8600lb with both. I have a 99 2500ccsb 8600lb truck with a FF. My buddy has the same truck with a SF. Every 3500 I have seen has been a FF regardless of whether its sindle or duals.
Same as with the gmt800s my 2001 2500 burb is FF. Guy up the roads is SF. Only difference I know of is mine is the 8.1 his is the 6.0.

 

[usanumber1]

Its a crapshoot on the gmt400 2500s. There were 7200lb and 8600lb gvw trucks. I have never seen the 7200 with anything but the SF axle. I have seen the 8600lb with both. I have a 99 2500ccsb 8600lb truck with a FF. My buddy has the same truck with a SF. Every 3500 I have seen has been a FF regardless of whether its sindle or duals.
Same as with the gmt800s my 2001 2500 burb is FF. Guy up the roads is SF. Only difference I know of is mine is the 8.1 his is the 6.0.

This is my experience with them as well. There seems to be no rhyme or reason when they used 10.5" 14 bolts and 9.5" 14 bolts.

 

[lilkia]

This is my experience with them as well. There seems to be no rhyme or reason when they used 10.5" 14 bolts and 9.5" 14 bolts.

The only common denominator I can find on the 8600lb 2500 trucks is the engine size. It seems to be broken down by small block (5.7, 5.3, 6.0) have had the SF and the big blocks (7.4, 8.1) have the FF. Again this is only from what I REMEMBER seeing I havent found any documentation either way. Dreis gmt900 2500 burb has the 6.0 and FF but it was a LE order vehicle so that may be the exception or maybe all gmt900 2500 burbs are FF.

It depended on the GVW you ordered. Regular cabs usually got the 6 lug "heavy half" 9.5 SF

Thats because the reg cab gmt 400 2500s were all 7200lb gvw or less so they would have the SF whether they were 6 or 8 lug. As for the gmt400s Im pretty sure only 2500 ccsb (cclb were 3500) and burbs were 8600lb gvw.