Chevrolet Repair: 88 Chevy 1/2 ton 2wd 350 TBI - hesitation, throttle position sensor, fuel pressure regulator

Question
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Followup To
Question -
Hi Van,

I have an 88 Chevy 1/2 ton 2wd 350 TBI p/u that hesitates.  Most often this happens when in Drive, under load going up a hill.  Instead of slowly accelerating as I apply throttle, it slowly loses throttle until finally it kicks down abruptly into second and gains throttle all at once.  In the snow, I find the wheels cut loose and start spinning as this all happens.

Additionally, the engine will sporadically begin to run rough, lose power and idle drops off....

I've checked the EGR valve which seems free and has vaccuum.  The fuel pump was replaced about 8 months ago.  I've changed the oil & filter, fuel filter, new plugs....

Any suggestions?
Answer -
Hi Bob,
The EGR valve should NOT have vacuum when it is just sitting idling. There are only specific times when it should open.
If it has vacuum all the time, I would try plugging that hose, and see if it helps.
The TPS, or throttle position sensor is what tells the computer to add more gas as you press the throttle pedal, and if it isn't working correctly, it can act as you describe.
Low fuel pressure is another possibility. That can be caused by the pump not supplying fuel, dirty filter, or faulty fuel pressure regulator.

Van

Thanks Van,

One more question if you don't mind.  Where exactly do I find this throttle position sensor?  I'm assuming the fuel pressure regulator is on the throttle body....

Thanks,

Bob Soper

Answer
Hello Bob,
The TPS is on the opposite end of the shaft that controls the throttle plates down in the throats. Opposite from the throttle cable linkage.
The Pressure valve is in, or actually on the throttle body.
On later models they provided a test port to check the pressure, but on yours you need to engineer a "T" into the supply line, or get an aftermarket test adapter.
I have one that goes into the connections where the filter goes, but I am hesitant to use it on a vehicle that may have dirty fuel, since it replaces the filter with a wide open "T".
I have considered buying a new filter, and soldering some connections to it, one on the pump side, and one on the engine side, or just one on the engine side, so the engine doesn't get a slug of unfiltered gas.
At any rate, if checking the pressure, you want a good full 13PSI, before it starts, and while running. It needs to stay at or real near 13PSI, even at full rpm, and full load.
A test of the regulator is if the pressure is low, but restricting the return line to the tank brings the pressure up to 14 or so, the regulator is bad.
If restricting the return line has no effect, it is probably not the problem.

I have had your problem where the pressure was 7PSI, but the pump sounded good.
When I went to replace the pump, I thought I would just do a little test of the pump, so I used a bucket of diesel fuel, and connected the pump to power. The pressure hit 7PSI, and fuel started spraying out through a little split in the hose that connects the pump to the fuel line inside the tank. When I plugged the split with my thumb, the pressure jumped right up to 14.
I replaced that little line with some fuel hose, and it is running great now.
Before that, it would idle fine, and sound fine till you took off, or needed power. Then it would buck, and jump.

Hope there is something here that will help you out.

Van