orthoxstice Reader
5/29/23 6:21 p.m.

I sold my beloved IDI F250 flatbed after the cab rotted out so badly that the doors wouldn't close. I had an eye out for a new truck for occasional boat-towing, mulch hauling, and craigslist finds and found this two-owner '89 dually. My IDI got 15-16 mpg with the 5-speed and this Chevy is a 454 turbo 400 truck and gets... maybe 7? But we use it so infrequently that I don't mind. 137k miles and a sticker valid until 1/2024. I paid right around $3500 all-in. 

Here's the day I picked it up (note the ladder rack):

I used it pretty much right away for a mulch run and for picking up some rocks for a pond project:

But then there was a twist; I street-parked it for two hours and it was stolen. Turns out the power locks don't work on the passenger side. It also turns out the ignition is worn-out and you can start the truck without a key. Thankfully it was recovered six blocks from where I'd parked it with less gas but more stuff in the cab; a mini-sledge, some fabric shears, a handful of empty Bic lighters, a crack pipe, and a receipt from the scrapyard. Seems the thief spent the night scrapping up a storm (he got $46). Unfortunately he was not careful with the truck and caused this: 

Ouch. This truck's paint is shot but the body is surprisingly straight for a western PA truck, or it was, until this. I'll have to keep an eye out for another fender. 

So far I've given it an oil change and changed the water pump. The truck came with a fan clutch delete and I wonder if that contributed to the water pump failure, picture here: 

The only issues I've had so far are the power-steering pump groaning and the steering getting sluggish randomly. No leaks that I can see and the hydroboost seems to work perfectly. Something to keep an eye on. The truck also misfired lightly when driving in a rainstorm so it may need an ignition tune-up. 

A 401 CJ
A 401 CJ SuperDork
6/18/23 7:48 a.m.

Cool.  I love that body style.

A cautionary tale on the 454: when I was a teenager our neighbor, a gentleman farmer, snagged a brilliantly gleaming square body 3+3 dually.  It was replete with dozens of chicken lights and a sprint-car wing as was common in the day.  "Built" 454 with headers and every trick done to maximize torque.  Farmer loaded up his horse trailer and headed for town.  I never saw the truck again.  When I asked my dad about some time later, he said that our neighbor had quickly sold it.  The reason?  It did 3, THREE! , MPG with the horse trailer.  surprise

Patrick MegaDork
6/18/23 8:47 a.m.

I DD'd a 90 3500 extended cab dually and got around 8mpg regardless of towing or empty. Great truck!

Floating Doc (Forum Supporter)
Floating Doc (Forum Supporter) UltimaDork
6/18/23 9:45 a.m.

I had a 73 with the 454, apparently the switch to TBI in the GMT400 didn't make the big block much more efficient than a quadrajet. 

Still, nice truck!

orthoxstice Reader
6/19/23 5:41 p.m.

Gas mileage is definitely terrible but since we only use it twice a month or so, I accept it as the cost of doing business. I have neglected to get anything done on this truck because my daily 940 Turbo needed a head gasket and new turbocharger taken care of. I've also had lots of house and yard responsibilities. So far all I've managed to get to on the dually is to repair the tape deck. I pulled the deck apart and put a new drive belt on and put some clock oil on all the moving parts and it now works flawlessly!

Unfortunately the speakers are so bad it hardly matters, but that's on the agenda. 

In the meantime, the C3500 makes a great mobile workbench:

bumpsteer Reader
6/19/23 10:15 p.m.

I am finding it amusing that the stories of 454-powered GMT400s getting the same ~8ish mpg fuel mileage regardless of if loaded or not tracks with every other story of 454/8.1 Vortec truck ownership I have heard IRL so far.

orthoxstice Reader
6/20/23 9:18 p.m.

Today I decided to eliminate the fan clutch eliminator kit that was on this truck when I bought it. The PO proudly informed me that the truck had a "four core radiator" and "no clutch" and that "it will never overheat"... makes me wonder if there was a story there. Anyway I figure fan clutches were a neat invention for noise and efficiency reasons and that I should put one back on. I had to travel hither and yon in order to get mounting hardware which was annoying but got it all mounted up:

And compared to the no-name eliminator fan that was mounted prior: 

That 11-blade plastic fan moves a LOT of air when the clutch is locked so I do not anticipate any temperature problems. 

Next up is to address the erratic, hunting idle the truck developed in the past week. In gear it seems fine, but in park or neutral it is all over the place. 

Floating Doc (Forum Supporter)
Floating Doc (Forum Supporter) UltimaDork
6/20/23 10:31 p.m.
bumpsteer said:

I am finding it amusing that the stories of 454-powered GMT400s getting the same ~8ish mpg fuel mileage regardless of if loaded or not tracks with every other story of 454/8.1 Vortec truck ownership I have heard IRL so far.

I did better than that with my 73, usually 12-13, with 16 on one tank being my best. Of course, the National speed limit was 55 then. I found 54 to be the ideal cruise speed. The longest trip, I drove straight through from Hamilton, Ontario to Orlando.

orthoxstice Reader
6/21/23 8:29 a.m.

It would be different if the speed limit was still 55, but this thing can just barely do 70 with the way that it's geared. I can't imagine driving it to Florida!

orthoxstice Reader
6/22/23 3:28 p.m.

So I drove the truck yesterday and it was really bogging and bouncing the idle, especially in gear. It was the warmest day that I'd driven the truck so my first thought was maybe the coolant temp sensor was bad but it seems to test fine. I wondered if it was a vac leak and used the old propane torch to check; couldn't find any. Then I remembered the PO telling me a story about chasing a misfire and how he had replaced all of the ignition parts (distributor included) and rebuilt the TBI unit. I got to thinking that perhaps things had not been put back to spec and my suspicions were correct. 

Base timing with the connector out looked to be 2-3 degrees ATDC so I set that back. 

I hooked up my old Sears dwell tachometer and with the IAC run in and unplugged it was sort of holding an 850-900 rpm idle. I adjusted that down to 700-750. 

TPS showed .44 with the throttle closed and I adjusted that to .53 or so.

After all that I went for the drive and the truck seems to drive perfectly fine. We'll see over a longer term if other things are going on but that felt like good progress. 

orthoxstice Reader
6/23/23 10:37 a.m.

One last thing in the engine bay; I was very displeased with how the plug wires had been routed. You can see two of them were rubbing on the steering shaft and it was overall a huge mess. I went to reorganize them and didn't manage to get a single one off without pulling the end off. After a trip to the parts store and liberal use of zip-ties as wire clamps I ended here:

Much better.

orthoxstice Reader
7/9/23 8:58 p.m.

Girlfriend and I used the truck today to move her new greenhouse to the garden and then as a workbench to assemble it and a new issue has arisen. The brake lights were on after I parked the truck and shut it off. I walked back over to the cab and pulled on the pedal and it moved... maybe a 1/4"? And the lights were off. Anyone ever have a brake pedal fail to return all the way? Is the return spring worn out? Master cylinder piston sticking? None of the wheels felt hot and I didn't smell dragging brake smell but I definitely don't want my lights to be on all the time. 

Though maybe that's better than the old timer I was behind in traffic the other day in an '88 or so 1500 with no brake lights at all...

orthoxstice Reader
7/13/23 8:33 p.m.

Still haven't figured out this sticking brake thing; unfortunately the rough-running condition returned and even got worse. The truck would buck like crazy and sometimes die at stops. 

After scratching my head for awhile and doing some thinking I recalled something from early in my use of forums back when I was on the Scirocco section of the VWvortex trying to figure out how to make my 16v run properly. There was a guy that would always swing into electrical problem threads to "spread the gospel of the free electron" and talk about how "current wants a clear path!" 

Anyway I checked my grounds. An OBS big block truck should have a few of them: battery to engine, battery to fender, firewall to frame, engine to frame. Then the harness grounds at the water neck. My truck had two intact grounds, one between the battery and the block and one between the block and the frame. The ring terminals at the water neck were there but the wires had been reduced to a few measly strands the color of the statue of liberty. There was no sign of the body to frame ground anymore. 

Anyway, I replaced all that and so far the truck does not randomly buck and die. Also the radio plays much better. Also the temp controls don't run away on their own.

All that's to say, I've joined the church of the free electron. Check your grounds, gang. 

dj06482 (Forum Supporter)
dj06482 (Forum Supporter) UberDork
7/14/23 3:24 p.m.

Also check your negative battery cable for corrosion. On my 1/2 ton GMT400, mine was shot and gave the same symptoms you're describing. On that model, it's very close to a lot of heat, so they degrade over time.

orthoxstice Reader
7/14/23 3:38 p.m.
dj06482 (Forum Supporter) said:

Also check your negative battery cable for corrosion. On my 1/2 ton GMT400, mine was shot and gave the same symptoms you're describing. On that model, it's very close to a lot of heat, so they degrade over time.

I replaced that one as well! Both as a "while you're in there" thing and because the integrated 8 gauge wire to the fender had broken apart. I figured that was a good indication of what likely lurker beneath the wire insulation. 

Used the truck for truck things this morning and ran smooth as glass, so I'm feeling positively about this whole experience. 

buzzboy SuperDork
7/14/23 5:03 p.m.

Kinda mind bottling that you could still get a non-overdrive trans in the GMT400 body style.

Cousin_Eddie (Forum Supporter)
Cousin_Eddie (Forum Supporter) Dork
7/14/23 5:11 p.m.

In reply to buzzboy :

I had a 1990 K2500 with the SM465 manual trans. Also the 1990 454SS trucks came with a Turbo 400. So, 1990 was the last year. Finally, in 1991, everything got overdrives.

orthoxstice Reader
7/15/23 11:08 p.m.
buzzboy said:

Kinda mind bottling that you could still get a non-overdrive trans in the GMT400 body style.

Agreed it feels very retrograde, just goes to show how forward thinking the GMT400 styling was in '88. My guess is the 4L80 wasn't ready yet and the only thing that could handle big block torque was the venerable turbo 400. 

orthoxstice Reader
8/5/23 9:25 a.m.

Used the truck to launch our sailboat of a similar vintage (very late in the season, but that's another story). I didn't really notice having the boat behind me at all while towing; plenty of torque to move it around with authority. 

Unfortunately there's still something... off about the truck. The idle has gone back to wandering around, especially when first getting moving. I'm running out of things to check. I do wonder about my fuel pressure since the po replaced the fuel-pump at some point. It looks like it has a new o2 sensor but I have one sitting on the shelf I could throw on just to be sure. 

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