91 Ls400 Rpm Pulse 1600-2100 In Park Also Bucks Around That Range Whil

[WHOOSH]

91 LS400 RPM PULSE 1600-2100 IN PARK ALSO BUCKS AROUND THAT RANGE WHILE CRUISING

IN PARK IT WILL PULSE BETWEEN THAT RANGE FOR EVER. IF YOU LET OFF THE THROTTLE IT WILL STALL OUT.RESTARTS IMMEDIATELY AND IDLES PERFECT. RUNS FINE ABOVE AND BELOW THAT RANGE.ONLY HAPPENS IF YOU TRY TO BRING UP RPM'S SLOWLY AROUND 1600 RPM IT STARTS PULSING TO 2100 BACK TO 1600 UP TO 2100 .... AFFECTS ENGINE WHILE OPERATING ON THE ROAD BUCKS GO ABOVE THAT RANGE OR BELOW IT IT RUNS PERFECT.ANY HELP ON THIS APPRECIATED !!!! TRYING TO LOCATE CAUSE. THANKS

[landar]

I would start by looking for a vacuum leak (loose or cracked vacuum line...even the throttle body accordian-style hose could have cracks). Rubber/plastic gets very brittle over the years especially with the heat of the engine.

What have you tried so far? Looked for any codes?

[kjhwv]

I had a problem with my 1993 cutting out at 4500 rpm's. Problem existed for 2 years and 20,000 miles. Car ran fine if I did not go above 4500 rpm's. Finally car go worst and it turned out to be the crank sensor. Changed it and everything is fine.

[WHOOSH]

Thanks Guys for responding.

landar the electrical engineer, so far I have acertained the functionality of the map sensor, throttle position sensor, all vacuum hoses have been replaced, I have be n refreshing everything that I see needed because I am planning on running this car forever. We love it. my sympathies for the one in your fleet that got totaled. I also replaced all the injector seals when I did the starter.I don't want to pollute your train of reasoning, but I am leaning toward fuel mixture problem possibly. just a guess.(when in park) when I feed the throttle slowly to raise rpm's response is fine until around 1,600-1,700 rpm then it has it's moment it wil pulse up to 2,000 - 2,100 rpm then back down to 1,600 then back to 2,000 for ever. give it a little more pedal and it will steady out at 2,200 and respond perfectly to red line . it also works perfect from 600 idle speed to 1,500-1,600 until it hits that panic pulse zone. if I let it pulse for a few cycles and let off of the throttle it will stall out.(turn key and restarts beautiful) if I let off throttle and tickle it a little it will steady out and stay running at smooth 600 idle. that is why I was thinking of a lean out condition? I bought the car with a replaced timing belt. so I opened it up to verify that the belt isn't off a tooth it inspected fine. while in there i inspected the caps rotors etc. all fine. the air intake has tight plumbing no leaks. question on the throttle body it has the butterfly that blocks off intake how does that work in the system. it is the one near the cruise control. Should I take the throttle body off it looks clean from the outside view but i heard the intake side might be carboned up. I havent checked the fuel rail pressure yet either but. Is there a possibility the oxygen sensors could affect the pulse problem? when you drive it it bucks every so often when it is operating at 1,700-1,900 rpm range below that perfect above 2,100 perfect plenty of power runs strong. Help i took the tps out and put in a new one no difference so i put the old one back in it checked out fine with the meter.I am a mechanical engineer and hands on type can take apart and rebuild manufacturer most anything feasible. thanks again no engine light on would i get any codes with out it lit?

[landar]

Even tho the CEL is not on, I would still check for codes. Can't hurt. If you are consistently hitting 'spots' in the RPM range that cause surging, it might be a sensor like cam or crank as kjhwv suggested.

[90Duchess]

At this point I'd do as Yamae suggests and look at bad ECU capacitors. It sounds like you are experiencing similar problems, and have tried to rule out the usual suspects. Check out the thread here: http://m.clublexus.com/forums/showpost.php?p=7605951&postcount=131

[lextechrob]

Always hard over the internet, but sounds like a weak ignition coil on bank1 (drivers side) usually. I have a seen three of them, first one drove me nuts. Load test the coil by pulling off the wire and watching the arc. it should be a very audible snap and blue 1/2 spark. compare left and rh. I use a spark tester tool you can get anywhere. Injectors which are fair game at this age and miles could cause a misfire but not likely a buck.