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poor running 95 chevy 3/4ton truck-350 with 5spd

S

SDE

Well-known Member
Hello, I bought a truck and trailer to haul my tractors. Unfortunately, this truck will lose power when driving on the highway, but it never stalls. It might start to lose power at 40 mph and without moving the acclerator pedal, it will drop down to 30 before the engine kicks back in and goes back to 40mph and then it will do it again and again. I put the pedal to the floor once and when it kicked back in, I got it up to 90. Once when it lost power I pushed the clutch pedal in and the engine went down to a about 500 RPMs and then jumped up to 1500 before going back down. When I stopped moving it idled at a steady 750. Any idea on what I need to look at to fix this? I put some injector cleaner in it and sprayed the throttle body and changed the air filter so far. It does not do any of this until it is warmed up. Could it be a sensor?
Thank you
Steve
 
IIRC the throttle position sensor can make that happen if it is getting intermittant. (my best guess) Jim
 
Test the fuel pressure while under load.
Was is a lean back firing type of miss?
Try a new fuel filter too.
If it's extremely hot air temps, the engine under heavy load and 87 octane fuel used. The knock sensor will back off the timimg and power will drop, but this sounds to be more than backed off timing.
Do you have a scan tool and looked at MAP, TPS, timing, O2 voltage etc?
 


Change the fuel filter located under the truck, driver's side, on the frame. It's an inline filter.

Is it still running smooth when it looses power or is it missing?
 
Is the exhaust in good shape? If the exhaust is rusty and has leaks, especially at the manifolds, the Y-pipe looses its ground and the oxygen sensor won't work properly. You need a scan tool to verify the oxygen sensor voltage when it acts up. If you don't have a scan tool, hook a jumper cable from the oxygen sensor to the battery ground cable and see if it acts up. Exhaust leaks before the oxygen sensor will also cause what you describe.

Outside of exhaust issues, you are probably loosing fuel pressure or volume. That engine only runs 9-13 psi of pressure, but requires a large volume of fuel to maintain power.
 
My 96 Chevy with 350 had a hiccup-at 45, slight incline, it would lurch under load. Rotor was bad, but also had to change the spark plug wires before it cleared up (I changed them seperately, drove it, so verified each 'fix').
 
how about just a simple tune up? plug wires, plugs,cap and rotor. go out tonight and start the truck, pop the hood and watch the light show from the wires
 
A coworkers first guess was a thottle position sensor, but he also thought it would only be bad in a smaller range of mph than the 8 to 10 drop I am exiperiencing. I intend to swap it out with one from my brothers truck, when he gets it back from a friend.
Thank you SDE
 
I looked at a couple of the plugs and they are not new, but they did not look bad enough to be a problem. I had the truck running when I sprayed it with carb cleaner. Both injectors looked to be working very well. I will check the rotor and the cap, if the rotor or cap were bad, wouldn't it be running badly all the time?
Thank you
SDE
 
I have already purchased a new filter, but I have not yet changed it. I went fishing tonight I it ran great. I suppose it could be a partially blocked filter.
Thank you
SDE
 
The manifold and front pipes look good. The muffler is not the correct one and it does not have a tail pipe. I will check all the connections and see if it helps. By attaching a jumper wire from the oxygen sensor to the battery will it cause the problem or will it cure the problem? A parts store man thought it may be low fuel pressure. I have an older truck that I could swap the gas tanks to see if the problem goes away, But it is a bit more work than it is worth if that isn't the problem either.
Thank you
SDE
 
I do not have a scan tool. I have borrowed one from the parts store twice. Both times it said there were not any codes. When it loses power it does not backfire. It feels more like someone turned the ign. switch off. The problem has only occurred when the truck is moving and so I don't know how I can check the fuel pressure at that time. But I sure wish I could.
Thank you
SDE
 
Had similar issues with two different fuel injected chevies and both times it was fuel pressure. Either fuel pump is getting weak or fuel filter is clogged.
 
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