Rpms Drop To Zero On Open Throttle

SuperCoupe400 · January 1, 2011

First - I've tried every way imaginable to do a search on this topic (fuel pump ecu) via that search window in the upper right hand corner, but cannot for the life of me get the search tool to yield a single result. I'll admit, I haven't been on this forum in a number of years, but this (new) search tool seems to be just plain FUBAR.

Anyway, here's my situation. I've been having rough, jerky, stumbling acceleration at low RPMs for the past 6 months or so. That plus the apperance of my check engine + trac control lights after punching the throttle to pass someone on the freeway led me to believe that my primary O2 sensors were going bad.

Then one morning upon starting my car ('93 SC4) the engine seems VERY restricted. I'd press on the throttle (still in neutral) and the engine response was like molassis - slow, lathargic, laggy. A pedal push that should have produced a 4,000RPM response only garnered maybe a 1,500 one.

Also, my the tachometer would simply drop RPMs down to zero the moment they hit 1,000! The engine keeps reving (albeit sluggishly), but the Tach needle only responds when RPMs are within the 1,000 range. Even on cold starts the engine idles at under 1,000RPM.

Additionally, I get some pretty moderate smoke out of the tailpipes.

I haven't even tried putting the car in drive. I'm afraid I may get it outside my garage, but then not be able to get it back in. I did change the primary O2 sensors. Also cleaned the IAC valve and blew compressed air through the MAF sensor. No dice.

Can anybody help me out here? I was considering trying a 12v Fuel Mod (bypassing the fuel pump ECU) - and will still give it a try, as the fuel pump ECU may be a culprit. But if it isn't, can anybody suggest other possibilities?

Has anybody else had these same symptoms?

Thanks

1990LS400 · January 1, 2011

I can't help you with your problem other than ask if you have checked for stored problem codes but I got 65 search hits on "fuel pump ecu" when I enclosed the words in quotation marks and searched the entire forum - not just the SC300/400 forum. There is an instruction on the search page: Tip: Search for a specific phrase by "enclosing it in quotes"

I not a fan of Lexus dealsers either, but you might consider looking for a competent independent Lexus mechanic if you can't get this figured out. I've been using the same independent Lexus repair shop since 1996 and I've been very satisfied with the service.

SuperCoupe400 · January 1, 2011

thanks. I'll search around for a Lexus repair shop.

I am, however, thinking about just biting the $125 - $175 bullet and getting the dealership diagnostic to pinpoint the problem, then just doing the repair myself.

therealtrikky · November 20, 2011

Let me know what you find out to be your issue! Mine does very similar, except mine will rev to 3k, then it's like you take your foot off the throttle, down to 1k.. back up to 2500-3k and back down.. 3rd time is a charm; it'll rev all the way to redline with no sputter hesitation. I've tested the MAF and verified fuel pump operation. Next I'll be testing fuel pressure and pressure regulator. With most vehicles this behavior is a MAF or fuel supply/delivery problem! I'm about to the point of taking it to a shop also. I'm running out of ideas and patience myself! haha