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Highway Surging/bucking Tracked Down


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I've been reading up on the surging/bucking at highway speed.... I have a bad case of it. My car has difficulty maintaining 70mph in overdrive due to misfire (which seems to cause the bucking) and also occasional part throttle acceleration drop off.

Unplugged and capped off the vacuum line to the EGR valve, and the problem has completely dissapeared. This is a super easy thing to check for.

I have not had the time to open up the ERG system and check it out, but running smooth and strong on the highway now. Might get to this today after drain/refill on transmission.


92 LS400 128k miles.

Just tuned up with Denso plugs, fuel filter, oil change.

Bucking and power loss at cruising speed only occurs when engine is hot.

Tuneup had zero effect on bucking.

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  • 1 month later...

I cleaned out my EGR valve housing, it was caked full of carbon. Air flows through it now. Same EXACT problem as before though.... on the highway car starts to surge at 65mph or so.

I have disabled the EGR to fix the problem for now. I was considering it may be an exhaust blockage which forces too much EG into the R :-) But really, it makes sense. Power still seems low at the high end of the revs. If there was a blockage it would raise the backpressure and cause too much exhaust to exit via the EGR system when it engages. The TCCS probably sees this, as a rich condition at the O2 sensor (one of the trouble codes I get). So it cuts back on the EGR VSV and the engine starts making power again.... then it re-engages the EGR and too much exhaust gets sucked into the engine, power drops off.... rich condition... seeing a vicious cycle here?

Only a problem at interstate speeds, where the exhaust volume is of sufficient quantity to cause increased backpressure.

Anyone follow this line of thought?

Please be something wrong with the exhaust.... I will order some rush headers and some raw tailpipe/muffler supplies the same day :-)


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