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After a routine tune-up, my Grand Cherokee would stall constantly at low idle and the acceleration would quit if I let up on the gas pedal right around 50mph. I had to wait til it dropped below 40mph, then give the gas a good hit. I'd feel a clunk and then a sudden forward burst of speed and it would run normally again until I let up on the gas pedal between 40 and 50. Also, the gas mileage has plummeted from 17-18 to about 10mpg.

The tune-up replaced plugs, wires, distributor cap, rotor button, pcv, thermostat and fuel filter. We replaced the TPS (throttle position sensor) and that didn't help. A friend zip tied the throttle cable, creating a bump stop, and that fixed the stalling idle problems but mileage and acceleration issue still present. I have the 5.2L V8 engine.

Was told it could be timing chain problem? This makes no sense to me. Please help!

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Let's start from the beginning. Why did you tune it up in the first place? If it was working fine before parts were removed and replaced, then something very clearly wasn't put back properly. –  hillsons Nov 19 '12 at 20:38
    
Agreed sounds to me like it could easily be a pressure loss somewhere, but with that amount of work down to the engine it literally could be a number of those things –  Chris James Champeau Nov 19 '12 at 23:05
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I ended up taking it to the Jeep dealership. Everything was replaced correctly and there was no pressure loss. The problem was in the Throttle Position Sensor. The auto parts store sold us a TPS that was supposedly OEM but the dealership said it wasn't. They put in their part and presto, a horrible engine was suddenly purring again.

I thought it important to post the follow-up because sometimes you do everything right and you don't think it's your materials that are the issue!

And btw, a tune-up (replace spark plugs, wires, etc) is not always done because something is running wrong. It's good engine maintenance, designed to make your vehicle last longer, be more efficient, and get the best gas mileage it can.

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Kate did you ask the original question? –  Larry Nov 27 '12 at 2:01
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As much as I hate paying the big $ for OEM parts vs. "corner auto store", it's often the right thing to do if you want a good solid, reliable repair. I've ended up with really early failures of corner auto store parts. Prime example was an alternator that only lasted 9 months, with a 1 year warranty. I fought them over it and eventually gave up. Never got a replacement or my money back. Went to the dealer and picked one up for 2x the money, but it's now been in the car for years just fine. –  Brian Knoblauch Apr 26 '13 at 12:05
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