Need Some Advice On Our Gmc 6000

[ThaddeusW]

Carnival season is about to start and this year we are going to fix up our truck mounted swing ride (half moon). The current truck chassis is a 1988 GMC 6000 with a 170HP carbureted small block that has been giving us plenty of problems. Since the truck weights about 16000 empty (guesstimate) the underpowered 350 and 4 speed cause the engine to overheat and run very hot. I have had one engine fire from spilled oil and twice had the carb rebuilt. The thing has dual air injection pumps and a maze of emissions hoses which have caused numerous vacuum leaks I had to fix. One hot day while crossing the tri-borough bridge I had to drop to 3rd and the engine was pulling hard, so hard it was starting to overheat and the carb went causing the exhaust pipe to turn into a flame thrower. I had the radiator checked and thermostat replaced but sill gets real hot. When you shut the engine down, its so dam hot it "diesels" and backfires so you have to be quick and leave it in gear and pop the clutch when the engine slows enough. It doesn't look or sound good when you pull up to a block party and the damn thing back fires and scares the hell out of everyone. It also burns rich and when you have to start the stupid thing to leave you wind up choking everyone out with the noxious exhaust.

I need to know if someone has experience with this crazy engine or an expert in chevy engines to give me advice (i know some of you have race cars, Larry ). No one has been able to solve the overheat (most likely because engine is over working) or clean up the exhaust. Also a new transmission would be great as 4 gears is simply not enough, I need a 5 or 6 speed truck tranny for a gasser. The brakes are hydraulic disc and rear axle is a single speed.

I have come up with a few options to solve the problem:

-Replace 350 engine with a big block setup for truck applications and 5/6 speed for extra gears

-Rebuild 350 for more power and torque and 5 or 6 speed

-Replace gasser with a smaller diesel like an old mechanical cummins isb or a detroit 6v53t

-"New" truck chassis like an old Mack B or R model with lowered suspension and tires (favorite option)

All have their pros and cons. A simple swap out to a bigger engine is likely the most cost effective option. Rebuilding the 350 is a bit out of my league but I have a family friend who does engine work with his brother, a drag racer. Swapping in a diesel will require extensive work including having to fab mounts, brackets and modify the fuel system. A restored classic Mack would be my dream but it will be costly and take a while to complete.

I might just try and shoot for an old Mack and restore the chassis then work the truck for this season with the GMC and a new/fixed engine. An old Mack painted up to look pretty will sell the ride and will look sweet pulling up to an event. Great way to advertise.

[Rob]

Thad, if it were me in that situation I think I would either swap the engine for a 427 "High Block" truck engine, or go with a 366V8. It sure seems the ole 350 engine is not up to the task for a long duration. A lot of the 427 engines around here had an SAE flywheel housing adapter installed and ran 10, and 13 speed transmissions also. You could mount an air compressor just to work the air shifted transmission and retain your hydraulic brake setup. I've done a couple actually in years gone past.

Either of those engines are relatively easy to locate around here, although they are getting older now.

Rob

[phantom309]

Upgrading to a Big Block Chevy like Rob suggested would be great. Sometimes you can find a 454 from an old motor home at a reasonable price too.

I suspect that much of the overheating problem you're experiencing is caused by a carb or ignition tuning problem. If the rad, thermostat, water pump etc. is okay two very common reasons for overheating are,, improper timing and or,, carb set too rich.

If the timing is not advanced enough, the fuel mixture will burn too late. That means that the piston will be a fair way down the hole while most of the fuel-air mixture is burning. That will expose the cylinder walls to more heat and that heat will have to be absorbed by the cooling system.

Too rich a fuel-air mixture will cause a similar problem. A rich mixture will take longer to burn so it will not only heat the exposed cylinder walls but will also continue to burn or be extra hot as it goes out the exhaust valves. That will of course expose the exhaust ports in the heads to extra heat which again, will have to be absorbed by the cooling system. The exhaust manifolds will also run hotter than normal, which will increase the under hood temperatures.

One other problem you'll run into is that the combustion chamber and especially the exhaust valves will get hot and cause both detonation and dieseling.

Jim

[maxidyne237]

Fantom 309 brings u some good points to check first. Also look for an vacuum leak from the intake or vacuum hose especially the hose going to the hydrovac usually under the driver side step. If the radiator isnt plugged try a high flow water pump ( a high perf one ) or at least a new gm one with a cast impellor because parts store rebulids use very crappy inefficient impellors. The first thing to remember is that that truck is never going to be a rocket ship but should do the job. If the 350 is in decent shape it should be ok. We have had c60 chevys before and the small block trucks can get the job done. I wouldnt swap to a big block. It isnt worth the work for the amount gained. You can put a five speed in that was an option on Chevy 50 and 60 trucks since the seventies. The best thing might be instead of a trans swap is putting in a two speed rear with a low ratio of slightly less than that of your single speed rear. Hope that helps

[Firemack]

Thad,

Just about everyone has owned and worked on the very dependable chevy small block, and I drove them in C50 and C60 trucks when they were new, and they could easily handle 26000-30000+ GVW, so it does not need a big block to handle the load. Everyone has provided a good checklist of items to go over and you should start with the basics -check the compression, vacuum and then tune up and replace all of ignition parts, fuel filters, and check the radiator, water pump, and thermostat, be sure to check for full flow. One other item to check is the muffler, it could be clogged and causing back pressure which can also cause the engine to run hot and lose performance.

The 2 speed rear is a good option and was on the trucks I drove, and the next best option is to add the 5 speed, but spend your money on the engine first and get it running right, then you can look at adding the options, as the truck as originally built should be able to handle the load.

Firemack

[ThaddeusW]

Thanks for the advice everyone. It does appear the 350 should be able to handle the load. The big problem is it sits and sits and sits, It has only 33k on the clock! When it goes out it does not go far, only locally so your lucky if you put 20-30 miles a day. Plus the fact that it does not go out every day during carnival season and it sits all winter long. Plus given the fact that until 2002 the truck has lived next to the ocean at my uncles house so the salty sea air doesn't help the problem either.

It definitely does not appear to run properly and takes a while to warm up. It actually has a choke knob, something I thought would be long gone for a 1988 when fuel injection was around. I just wish the air injection system wasn't there because it makes getting at the engine very difficult. I will dive into in the upcoming week and see what I can find.

phantom309,

It does run very rich, the exhaust smell can knock you out and it smokes and back fires a bit while warming up. Not loud but a puffing sound accompanied by a short dim flame from the exhaust. That muffler has seen allot of back fire and fire. The day the carb went on the tri-borough it would not start and I had to be towed home. the next day it started again but would quickly stall but it had to go out and we sent it out. Our driver thankfully is a veteran truck driver and when the carb started in with problems he knew how to get it going again. But it was shooting 3-5 foot flames from the tail pipe due to the carb float getting stuck flooding the engine with gas. We had the carb rebuilt but it still does not burn right even after sending it to two repair shops. I will look into this.