Problems with 1990 Chevy Tow vehicle

[Boatwrench]

I have been battling a problem with my 1990 2WD 3/4T Suburban, 454cuin (7.4L), THM400 3-spd auto, and 4.10:1 ratio. The odometer is pushing 151K. The majority of this vehicle's mileage is from highway driving. It was used for family vacations across country (2x coast to coast and 4x to midwest IA, MO & MN) from California. 50% of that time towing a 4k lb trailer.

On Sep 6, 2008 it ran hot (sans trailer) and needed a tow home. Checked it out and the Thermostat had stuck closed. I replaced the thermostat and the vehicle continued to run warm. I drove 1K miles round trip to SOCAL later in Sep. and it was warm, but not hot. The fan clutch wasn't working and there was weeping at the w/p. I said heck with it. Recored the radiator, replaced all the hoses, cleaned the fins of the condensor, oil cooler & trans cooler, flushed the heater core. replaced the w/p and fan clutch. Some where along the way the interior lights were left on for several days and the battery was replaced.

Getting ready for Fall Fling 2008 and it started running like crap. Low power on freeway, sluggish 2-3 shift, the timing was 12* advanced and not settled swinging from 6-12* BTDC, spec is 4* BTDC, set a code 43 (knock sensor).

Since then:
Replaced catalytic convertor. (8psi back pressure) 3" exhaust from collector on back. No change
Replaced Vibration dampner (had a squealing belt & harmonic vibration sound at freeway speed also) No change
Replaced Knock sensor (code 43) Still sluggish, doesn't set codes.

Have now gone back to basics: Mechanical, Fuel, Electrical
Yesterday:
Checked timing measures 4-6* BTDC (spec is 4*)
Manifold vacuum measured with very slight fluctuation (15-17).
Compression test:
1 = 155; 3 = 145; 5 = 155; 7 = 145
2 = 150; 4 = 160; 6 = 150; 8 = 150

Plug condition:
#2 & #8 sparkplug boots were torn.
#7 and #8 sparkplugs were dry fouled (soot or oil) electrodes were black, not the pink carbon color.
The plugs in the vehicle are AC-Delco CR43TS, what the catalouges list as the standard plug.

Air filter element clean. Has 2,500 miles on it.

Fuel:
Have not checked fuel flow or pressure yet. Vehicle has Throttle Body Injection and I can't find a tap to check pressure. In tank pump and sock. Fuel filter has 2,500 miles on it also.

Electrical:
I'll pick up wires tomorrow on the way home from work. The plugs? My Mechanics Encylopedia I can't locate but it had a great explanation of when to change the heat range of the plug. Any suggestions?

Thanks in advance,
Tom

 

[NW-ZJ-SCOTT]

You have covered every corner that comes to mind,

Is the chevy symbol on the grill still shiny?

 

[Boatwrench]

Is the chevy symbol on the grill still shiny?

Yes. This has been a garaged and pampered vehicle. Has small three dents, no torn upholstery. I bought it brand spanking new, only 17 miles on it. Had orders to the Persian Gulf and wanted something with a warranty for the bride and 4 small children.

 

[Johnnie Walker]

Pull that 454 out and throw in a caddy big block (=

 

[8Mud]

When you set the timing, did you disconnect the "to set timing" connector?
Are both of your injectors working? It will run with one plugged up, but poorly.
Whenever I see a rear plug, soot fouled, I think head gasket, but both 7 and 8 make this unlikely. A fuel issue? A rich motor tends to show up in the rear cylinders first, a lean condition in the front.
Off the top of my head I'd look for an intake or exhaust restriction (seems you have that covered).
Use a stethoscope and check for top end valve noise. If the valves aren't opening completely, this can soot foul your plugs.
Does the 90 have one or two knock sensors? Chev's like a lot of advance, but it's mostly noticeable after mid RPM range.
Accelerate up an on ramp with the motor warm, watch the CEL closely. There are intermittent codes that won't set, usually fuel related.
EGR? Any knock or stalling? If it isn't working at all it can also cause overheating.
I have 92 GMC fuel and emmisions service manual around someplace, if you need any info let me know (PM).