rbert

I have a 95 SC400 that has an off idle hesitationon (kinda hiccups) on acceleration about 1100 to 1900 RPM. It seems worse at outside air temps of 75-85 degrees. It runs fine until it gets up to normal operating temp, also runs fine under hard acceleration above 3000 RPM, below that seems down on power and have noticed some occasional light pinging and some surging at 45 MPH or so. I have added Chevron injector cleaner, cleaned the throttle body, changed caps, rotors, wires, plugs, checked fuel press at the rails (normal), checked VAF air straightener and for leaks downstream of VAF sensor, tested IAC stepper, new air filter, PCV, checked temp sensor and EGR. Checked the TPS rigging and potentiometer/switch tests per the Lexus maint manual (Resistance tests in the manual are wrong) so I ended up with a spare TPS. I fianally ended up taking it to a Lexus dealer where they changed the TPS which didn't fix the problem, obviously... I brought it to another Lexus shop where they changed the both main oxygen sensors, same problem. I'm assuming they checked the timing. Fianally, at the suggestion of a friend I disconnected the VAF sensor and the car runs perfectly. A friend of mine has the same car, same year so I swapped his VAF sensor and same problem (his runs fine with this sensor). With this (VAF) disconnected, does this take other sensors out of the loop? some other sensor bad? computer? This car is showing no fault codes and I don't have all the test equipment. I do alot of driving in traffic and this is a very annoying problem. Anyone have any ideas? or does anyone know someone good in the SF bay area? It seems if cars are not throwing codes, the dealerships won't troubleshoot beyond that. Any suggestions would be greatly appreciated.

Nomis

rbert,
I have the same exact problem on my 95 SC400 - what and where is the VAF sensor located on the engine? I'd like to disconnect it as well and check for any adverse problems. I'm assuming you have yours off right now, right, and no problems with it running?

LexAnt

iTrader: (2)

try fuel filter? i have the same problem and i hear the fuel filter might be slightly clogged.... so maybe?

rbert

The VAF is a volume air flow meter that measures how much air the engine is "breathing". It provides an input to the computer which tells it how much fuel to schedule to the injectors (along with other sensors) and also provides input for ignition advance. If you stand in front of the car looking at the engine, it's on the left side just in back of the air box (which is just behind the headlight) mounted on the top of an aluminum coupler connected to the 4" intake hose, it's about 3X4 inches by an inch tall. Mine is a Fuji electric P/N 22204-42011. Theres a tab you have to push down on the connector to disconnect it, it probably will be sticky coming off so work it back and forth carefully taking it off (don't be an animal). You will get a check engine light. The loss of this sensor will put it into a limp home mode where the computer will use some default parameters to compensate for loss of sensor input. You should probably disconnect the battery (neg) for a couple minutes before disconnecting/reconnecting the meter (as per the manual). Let me know how it runs. This problem seems to be a lean condition an it will burn more fuel with it disconnected (probably normal). I have my buddy's sensor in the car now and still runs crappy. As for the fuel filter, Someone overtightened the fittings so bad I had to take a breaker bar to get them loose and then couldn't separate the lines. The filters use a straight pipe thread male ferrule and the flares must be expanded into the filter housing, so I stopped there (can't deal with fuel running into my armpits) Probably have to change the lines out to fix it. So im sure the filter was changed by someone to troubleshoot this problem. Lexus said these are life time filters and they never change them? Whatever.....Anyway, I did T into the fuel rail and taped a pressure gauge on my windshield and drove around for awhile. Per the book, pressure was normal 38-44 psi or something like that, no dips so I kinda ruled out the fuel filter, especially because it runs great with the VAF disconnected. I bought this car used from a dealership in San Jose, Ca. with 55,000 mi. More later....

Nomis

I'm surprised its not throwing all kinds of codes with VAF/MAF disconnected.

rbert

When it's disconnected the check engine light will be on and it will generate a code. You need a code scanner to see what code it is, in this case the VAF.

ChrisK

iTrader: (1)

I would check for air leaks between the throttle body and MAF.

rbert

I checked the ducting/plenum and rubber coupling for cracks but didn't see any. The only spot it could be leaking is where the lower part of the plenum is glued on. I'll try pressure checking it. Im still wondering why it runs so good with that sensor disconnected?

ChrisK

iTrader: (1)

To me it looks like the MAF sensor is not reporting the proper amount of air. When disconnected the car relays on different sensors/defaults. MAF is one of the main sensors that ECU uses to calculate the air/fuel ratio.

It looks like you tried a lot of different things, pretty much everything that I could think of. Here is a wild idea for you, try using your friend's ECU (make sure they have the same part number first). If the hesitation still exists, I would recommend starting from the beginning and recheck the spark plugs, cables, etc. Maybe somebody else has the better ideas.

rbert

I've been thinking about doing that but wasn't sure if there was the possibility damaging his ECU? Like an out of range/overvoltage sensor output from my car? Lemme know what you think. Theres a Lexus special service tool (jumper) that T's in at the ECU to test sensors, but kinda tired of throwing money at this thing. Also seems like it effects the shift schedule, you step on the gas and it shifts right into 2nd gear sometimes. Thanks for the replies.

ChrisK

iTrader: (1)

In this case, what about testing your ECU in his car?

rbert

Im going over there tomorrow to give him back his mass air sen. I'll swap it out and see what happens.
I need to drive it for a day or so to really see if it fixes it....I cant belive there arn't more posts for this same problem.

akforte

perhaps your fuel injectors are dirty?

UpInTheLex

iTrader: (5)

95 SC400, same problem. Hickups at low rpm, taking off from a traffic light. until 1900 rpm. I think it's plug wires or cap/rotor. I have clean injectors, new plugs so it's not that. I doubt a sensor is the problem becuase it would set a code. Torque converter maybe is goofed up, I know the stock stall speed is 1900rpm.

I want it to go away.

rbert

Well that wasn't it. I swapped ECUs with my buddys car and same thing. Think I need to revisit TPS rigging and ignition sys.