Rough Idle, Vibration in reverse gear and taking off. Help!

[ron0429]

So i drive a 93 xlt its all stock only 101,000miles, I hopped on county highway in front of a semi coming faster than expected so i got on my gas pretty heavy. usually the rpms will get going when i punch on it but this time the rpms kind of just took their time climbing. it didnt matter if i pushed down any harder they were just going up at their own slow steady pace and i have noticed this happens quite frequently. its usually 50/50, speaking about when i punch the pedal the truck will either seem like it has more power on the acceleration and climb through the gears easily or it will kind of "lag" up to speed as i just mentioned before. but this time is now different. after i punched it in front of the semi and came to a stop-sign, i noticed a vibration when i took off (almost like i was going 5mph over rumble strips or it almost felt like the car was going to stall) This goes away after 5-10 mph and the car drives and shifts fine after that. I've also come to notice now that when idling the car rumbles a tad bit like its off timing and also my reverse gear does the same vibration as accelerating causes. I'm guessing it could be a transmission problem. If anyone who knows about this kind of stuff could help with some info/troubleshooting it would be very appreciated.

-Thanks!

 

[awood]

Check the sticky's here for pulling codes. Especially the procedure for the KOER test. There's a part of the KOER test that's called a "goose" test that checks the Throttle Position Sensor (TPS) when you stomp on the throttle. You just may find that it's faulting the TPS. I had similar issues with a Ranger (same engine), and I had no check engine light. Might be worth a try. The KOER is a dynamic test with engine running, but you can also search for procedures for a TPS static resistance check with the engine off. Just search for a TPS test using an ohm meter. I've found that for this test, an old analog meter is better visually than a digital meter, unless you have a digital meter that also displays an analog bar across the bottom of the display.

It's a fairly painless (and free) check. You may have other issues causing the problem, but at least you could rule out the TPS.

http://oldfuelinjection.com/?p=94
see step #9

 

[arco777]

I doubt your transmission has anything wrong with it. As awood stated, check codes first. Run KOEO, KOER, and I would suggest a Cylinder Balance test.

You may actually have a misfire. A misfiring cylinder will cause the engine to shake even at idle, and heavily under load. Misfires due to weak spark sometimes go away when the engine is hot. Open the hood at idle. The engine should not be shaking at all, and the tach needle shouldn't wiggle at all, just stay steady. Should sound perfectly smooth. If it's not smooth and steady, a cylinder is likely misfiring. There are many ways to pinpoint which cylinder is misfiring. You can check temperatures on the exhaust header with a infrared temp gun or splashes of water (cooler spot indicates exhaust outlet of the misfiring cylinder). Another way is to unplug one spark plug wire at a time while idling, and see which one has no effect on the idle quality. If you can identify which cylinder is misfiring, you can verify the three things needed for it to function: fuel, compression, and spark. All three are fairly easy to test.

Another thing to check is your fuel pressure. You can buy or rent a test gauge and connect it to the test port on the fuel rail to verify whether your fuel pump is supplying enough fuel, and if your regulator is working properly. You can even leave the gauge connected and drive the vehicle while watching the gauge to see how the pressure holds up. Problems with fuel delivery are usually due to a failing fuel pump, clogged fuel filter, faulty fuel pressure regulator, or faulty injectors (rare).

If you have questions about any of the procedures mentioned above, try doing a Search on the forums by using the Search box at the top or bottom of the page. Most questions have been asked and answered many times before, and there are many helpful how-to guides posted on the forum (like how to pull codes).

Finally, welcome to the forum!

 

[ron0429]

Thanks a bunch for the quick reply guys. Turned out to be a bad spark plug and the wire wasn't connected which had the cylinder out of action. She got new plugs and wires and accelerates as it should now. all better

 

[awood]

Thanks for the post of the fix. Sometimes we get caught up in the (several) unique problems for the 4.0L engine that we forget the basics.