Truck runs pretty good when first started. Acceleration from stop, and when increasing from 25 to 40 is acceptable (for the weight and drag from ladders). However, when the engine reaches operating temp, the performance drops. The hotter the engine gets the worse the performance. On hot days w/ the temps in the 90's, the performance is so poor that even w/ accelerator pedal floored, I sometimes have to turn off the a/c to get any acceleration from a stop. In any case, once the speed gets to 40-50 it seems more "normal". Never "peppy", but it IS heavy w/ lots of airstream interference.
The other anomaly that is interesting, but not exactly correlating to the performance is the instrument cluster temp gauge. Sometimes it works and sometime it doesn't. Often when it does work, it is during the cooler weather. When the engine (and pax compartment) is fully warmed, often but not always, the temp gauge will fall down to fully left deflection (cold). This fall is after the performance loss has occurred. Sometimes the gauge never moves at all, just stays fully to the left. On a few occasions it will perform normally all day. Remember we make several service calls a day in this truck, so lots of starts, short trips, sit an hour or two and repeat. I can't correlate the temp gauges movements to anythig else! ( Yes, heat works fine in winter. We've changed the stat 3-4 times.Cooling system is flushed every other year MAX. Often EVERY year) Check engine light does not show. Yes, the lamp comes on w/ key on but not yet atarted.
I've had this complaint to 2 different independent shops and 2 mobile diagnostic visits. All 4 times there are NO codes in the computer to indicate where this might be. When the vehicle is tested by our clean air Nazi's, it ALWAYS receives a passing grade. Not sure what they check for in my part of Missouri, but for the last 6 years it has always been passed on their "fast pass" parameters. (Meaning very low emissions, I guess)
Several years ago, first shop replaced plugs, wires, dist cap, rotor? and anything else that has to do w/ "tune-up". It made absolutely no difference. Neither independent shop has any helpful suggestions other than, "well, we could try changing the (pick one) sensor and see what happens." I remind them IT'S A 90 w/ MORE THAN 100,000 miles. I can not afford to rebuild the damn thing!
So, here we are. Anybody had a similar experience? Looking for various suggestions that I can try myself. Take it to the dealer and spend $500-$1000 is not one I'd think makes much sense.
Thanks, Gary Kasten