wilshire

Update: All I needed was a new set of spark plugs.

This issue has been annoying me for months now, and with the weather warming, I'm hoping to get it sorted out.

When I drive on the highway, my truck likes to 'kick' a little bit. It's worse when I'm trying to go faster or up a grade. If I'm semi-coasting, no bucking or jerking. If I don't go above 60, I can mostly keep it from doing it at all. It seems like I can slowly climb steep hills with my foot in the gas all day long without feeling the buck - it's the long 5th gear grades that are the problem.

It's a bit intermittent. Sometimes I can charge up a grade at 70 MPH that would often give me problems and not feel it jerking at all. Other times I nearly pull over on a fairly flat stretch that doesn't require much throttle and doesn't usually present a problem.

What's puzzling is that this seems to be the only symptom. I'd expect problems with fuel delivery to be tied to fuel demand, I'd expect ignition problems to be tied to RPM. You wouldn't know there was anything wrong with my truck if you never drove it over 50. No codes, runs strong, idles fine, just bucks on the highway under load.

Yesterday I thought I had a breakthrough when I read that the ECU gets a signal from the vehicle speed sensor and if the sensor is bad the signal can trick the ECU into thinking the vehicle is going way too fast and to cut fuel - intermittently if the bad signal is intermittent. It would tend to only happen at high speeds, you'd expect it to otherwise run fine, and you'd expect no codes. Exactly my symptoms. Excitedly I unplug my speedometer cable in the parking lot at work and feel for the jerk as I drive home. At first I was sure I'd finally solved it. Success, no bucking. But then, there it was, same as ever.

It's really getting annoying, and I want to know I'm not hastening the demise of my truck, driving with a dangerous condition, or ignoring something that's going to leave me stranded. Summer's coming and I need to be able to drive.

Once for the heck of it I unplugged the TPS and went for a drive. Truck was a bit less throttle responsive but the highway jerking was still there. Does this rule out bad data from sensors?

Plug wires are NGK, fairly new, and within ohm spec. Distributor cap, rotor, pickup coil seem good. Ignition coil tests fine.

The fuel system I have no idea about.

If it's the engine malfunctioning, my plan of attack is straightforward:

- Adequate fuel pressure at the injectors must be maintained
- The computer and sensors must be properly signaling the injectors to pulse, the injectors must be receiving that signal, and properly pulsing
- Spark must be adequate and timely
- The flow of gasses into and out of the system, including emissions stuff, must be correct. This includes correct valve operation.

If it's the engine, any problem it's going to have will fall under one of those four, yes?

Is there anything other than the engine? It's really got me puzzled that it runs just fine until I get going fast. I can rev it in neutral all I want and it runs just like it always has.

Very strange, very annoying, please help me get rid of this terrible jerk if you can...

OrRunner

iTrader: (1)

Being that you unplugged the TPS, and there was no change, I would think it MIGHT be ok. I would check resistance values on the tps with a meter, then I would start checking things like the o2 senser, fuel filter, and FPR. My guess is the tps or O2 sensor.

wilshire

One other thing. I changed the timing chain last July. I can't be sure when this issue started, as it can be subtle enough that I may not have noticed it at first driving on our bumpy roads here in PA, but I know I knew something was up by September. I'll be pulling the valve cover and checking the cam timing tomorrow. Could the cam being off a tooth cause this sort of thing?

Dirt Driver

My truck had a subtle bucking at low throttle when the O2 sensor was on its way out. I think a CEL only popped up maybe twice in the several months that the condition surfaced. I don't remember exactly; it was several years ago. More than once were there no trouble codes.

Was there a CEL with the TPS unplugged? If not, there should have been.

wilshire

There was no CEL with the TPS pulled but it did cause a code when I checked for codes later.