emLEX

Kind of got a lot of key points, so I'll leave out the small talk in an effort to keep things uncluttered.

At low RPM, below 2k, centered around 1500 and at light throttle, the car hesitates and stumbles.
Does it both from a stop or rolling, like cruising a parking lot looking for a spot.
Does it in both 1st and 2nd gear, don't think it's transmission related, but...I don't know.
Does it only when in gear/load. Cannot replicate in park or neutral.
Does it only once engine is warmed up. If I start driving it before it's warmed up and the idle is higher, it's fine. Idle cold is at about 1200, comes down to about 550-600 once warmed up.
Car starts and idles great.
At medium to wide open throttle, no issues.
Above 2k RPM, no issues.
No codes
All ignition parts replaced(plugs, wires, caps, rotors, coils) - no difference
Fuel pump ECU bypassed - no difference
Tried a couple of used TPS - no difference.
Did the Seafoam thing - no difference.
Had a leaking power steering valve into the intake, it's now been bypassed - no difference.
No vacuum leaks.

EDIT: 3/20/16
Replaced fuel filter - no difference

EDIT: 3/30/2016
Replaced engine temp sensor - no difference
Replaced both upstream O2 sensors - no difference
Replaced ECU speed sensor with a used one - no difference

Anyone have a similar issue? What was the fix? I've got a fuel filter coming and I'll be cleaning the throttle body this weekend. Aside from that and what I've already done, I'm at a loss.

Thanks

t2d2

iTrader: (8)

I'd be shocked if it isn't ignition coils, but since you already replaced them...did you go with OEM?

emLEX

I did. Almost every part I've put on was OEM with the exception of the ignition wires which are Belden "Edge Premium" wires from NAPA and the distributor caps and rotors are Bosch. I did all of the ignition parts at the same time as the timing belt, tensioner, idlers, water pump, crank seal and cam seals(all OEM). All of that cost a decent penny so I went a bit cheap only because RockAuto had them on closeout for 8 bucks.

emLEX

The new OEM fuel filter came in and I installed it yesterday, no difference, still stumbles and hesitates as described in the first post.

I'm at a bit of a loss here. I will probably try a new TPS, although I'm skeptical of that being the problem considering 3 different used parts all had the same issue. Guess it could also be O2 sensors, just thought I'd get codes for that.

Coleroad

Have you looked at engine temperature sensor. Intake air temperature sensor, ambient temperature sensor. Live read them and see. I would take a bet on the engine temperature sensor. It uses a different sensor for the computer than the dash gauge.

SC400slide

iTrader: (11)

ECU...ECU...ECU...not the fuel pump control ECU, main engine ECU.


Rebuild, will run you around 110 bucks from Tanin Electronix here on the Club Lexus forums.

emLEX

I have not. Is there a link or procedure explaining what to check and what the readings should be?

emLEX

While I'm not ruling it out, every symptom I've read about bad ECU's is different from what I'm experiencing. Also, I thought this was a problem for earlier model years.

Can you explain why you think it's a bad ECU?

burteboy

does this condition only happen in warm up? if so it could be a bad o2 sensor.

SC400slide

iTrader: (11)

If you haven't touched it, or know it has not been touched for repairing purposes, then it likely needs a rebuild. It's not the miles. It's not the conditions, it is more the age. It is more prevalent in the earlier model years due to the excessive age on the circuit board components. (Capacitors/resistors/ect) However, 21 years is a lot of age on a circuit board.

Also, you mentioned that there is no CEL. ECU's aren't made to detect problems in itself. Something is obviously wrong, and a check engine light should be on, but there isn't, so the ECU is not diagnosing something that is malfunctioning. Something else to check is if the CEL works if at all. Turn the key to "on" and all of the cluster lights should turn on. If it doesn't, the bulb may be out. Just to make sure that there isn't a CEL, remove the cluster and swap the bulb out of the, say, door ajar light, or some useless indicator. Just make to be sure.

Something else to keep in mind is there are quite a few different capacitors on the board that are made to different parts of the car. They all are the same age, so each one will go out eventually as nothing lasts forever, so in an instance of a bad ECU, symptoms can be all over the place. You can have all sorts of weird issues happening.

In the end, if the ECU does NOT fix the problems, at least you know it is rebuilt now and the car can be tuned correctly. My SC was running fairly well and I sent the ECU out to be rebuilt. Though, rebuilding didn't fix the main issue I was looking to correct, the individual who serviced the ECU said he was surprised the car was even running. Hope this makes sense.

emLEX

Yes, only when warmed up which is why I was leaning towards the O2 sensors. My theory is that while the engine is cold, it's in OPEN loop, meaning it ignores data from the O2 sensors. Once warmed up and it goes to CLOSED loop, it's now relying on the O2 sensors and if those were going bad...

This also correlates to the above mentioned coolant temp sensor, since 1 of the parameters necessary to switch from open/closed loop is coolant temp. However, it's my understanding the ECU only uses coolant temp to determine if the engine is at the proper temp to switch loops, not necessarily to adjust fuel/air mixture...I could be wrong about that.

emLEX

Yes, it makes sense. I come from an electronics/audio background and bad caps is a common fault primarily due to age and the quality of manufacture. I've recapped speaker crossovers, fixed broken flat panel TVs, repaired dead amps... by replacing dead caps. I'll keep it in mind as a possibility.

burteboy

Your problem is not the ecu. Connect a tech stream and read the data while the engine is acting up. Look at the fuel trim and c if its running lean, look at he temp sensor . Did you check your maf sensor .you need to look at the data to determine the problem. Guessing will only be expensive. Have some one putthe car in a check mode and maybe a code may pop up.disconnect the o2 sensor and check the vehicle.

emLEX

I didn't think the Tech Stream worked on OBDI cars.

Either way, I've decided I'm going to order the temp sensor and both upstream O2 sensors. Even if they don't fix my problem, considering they're likely to be 21 years old I'll look at it as preventative maintenance.

emLEX

Thought I'd bump this with an update, hoping someone will see it and have the answer.

Replaced Engine Temp Sensor...no difference
Replaced both upstream O2 Sensors...no difference
Replaced ECU speed sensor with a used one...no difference.

I ordered a new throttle body gasket so I can take it off and clean it thoroughly. I did clean it a few months ago, but with the TRAC throttle plate in the way I couldn't really get in there easily to do a good job. Also got a trans filter/gasket/fluid, so those will be the next items to get done.