myf16

My 1990 LS400 occasionally loses power for a fraction of a second, even at idle. However the idle is perfect in Park and Neutral. The problem only occurs in Reverse or Drive, stationary or moving, overdrive enabled or disabled. It also only occurs after the car is warmed up. The first few miles it runs perfectly.

Has anybody here had this problem? Any guesses are welcome.

If nobody has a better idea I will change the transmission fluid. It could be the original fluid with 140k miles and definitely has been in there for at least 65k miles. The fluid is clean, it does have a normal not burned smell, and the level is correct.

The plugs and coils are also likely original with 140k miles. I don't know why they would behave differently in D or R, and the smog test was perfect, less than 1% of the max allowable HC. That tells me ignition is working well. Right?

I drive this car less than 2k miles per year, and the body is kind of beat up, the paint is weak, and the A/C is broken. I don't want to spend time and money on repairs that won't be needed during the next 20k miles.

Incidentally this LS400 has the ultra-rare cloth interior, in gray, in perfect condition. Cloth does not age and crack like leather. I intend to drive it until it fails or someone smashes into it.

sdls

Back in my day people would go straight for the tune-up stuff.. you know, injector cleaning, spark plugs, wires, distributor rotors, caps, coils.
Nowadays people blame the ECU's capacitors for everything.
YMMV

myf16

I changed the transmission fluid but the problem persists.

FWIW this was the cheapest trans fluid change ever. I bought a 6-foot vinyl tube from Home Depot for the front left radiator output. I used 4 feet of that so the catch bottle would be visible to the left of the car. I bought a $4 funnel from Harbor Freight and notched the small end so it would be snug in the dipstick tube. I used all 10 quarts of Valvoline Import Synthetic Trans fluid that I had bought at Autozone on closeout for $1 each. The job took less than 75 minutes start to finish, including a failed attempt to remove the left front tire, which wouldn't have helped much anyway. Damn tire shops and their impact drivers...

myf16

This behavior is getting more pronounced, almost like the bucking I experienced in my Chrysler Pacifica when its cam position sensor died (which has now happened 3 times, with the third replacement coming this weekend).

The problem does NOT occur before the car is warmed up or when the transmission is in Park or Neutral.

Is cam position sensor failure common on the first-gen LS400? Do my symptoms fit?

Edit: I will check for error codes tonight and report the result here.

Edit2: I should have known that no Check Engine Light = no error code (for OBD1).

djamps

Sounds like a bad coolant temp sensor.

Sin1UZFE

YOU HAVE MY UNICORN!!!!!!! Lucky. But like others say sounds like a coolant temp sensor. Easy and cheap fix.

myf16

Please explain hoe the coolant temp sensor could cause this. My gauges are not usable due to capacitors. I have an infrared temp sensor which confirms 177 degrees F max at the thermostat housing, so not overhearing. No glowing exhaust or cats after a freeway drive.

One other clue. During that freeway drive I switched off the overdrive. Maybe it was only that the rpms increased to 2400-2500, but the problem virtually disappeared. Arriving home I noticed a very slight occasional miss at idle in Park, not just in gear. So it's not the overdrive engage/disengage command causing that.

I will order the coolant temp sensor on faith, but I'd like to know why.

Sin1UZFE

The logic being if the computer is getting wrong info from that sensor which it almost mainly depends on it would not know that it's sending the wrong fuel air ratios at any given temp. At any rate a good quality sensor runs about $30 and is easily replaced save for some coolant spillage.

At least that's my logic

https://www.lexuspartsnow.com/parts/lexus-gage-assy-water-temperature-sender~83420-16020.html?Make=Lexus&Model=LS400&Year=1990&Submodel=&Filter=()&Location=pd:,

The one for my 99 is 60 lol

bradland

Good job on the fluid exchange, unfortunately, you used the wrong ATF. The Aisin A341E transmission uses Toyota Type T-II fluid which is no longer available. Type T-II has since been replaced with Type T-IV. More importantly, the various Aisin transmission used in the LS line don't play nice with "Multi-Vehicle" (Honda Nissan Toyota ALL THE ABOVE etc) ATF's.

As for the misfire/loss of power-
Considering the age of the car, if the plug wires are in fact original I would look at them first. The symptoms you describe point to aged ignition components and, as mentioned above, bad ECU caps are always a possibility on the earlier models. The coolant temp sensor is important for proper fuel economy but the role it plays in engine management isn't powerful enough to cause a misfire or temporary loss of power.

Sin1UZFE

What do multi vehicle atf do to these trannies? Causes these erratic behavior?

myf16

I have not noticed any difference in behavior during the first 200 miles since the fluid change.

Sin1UZFE

Was the miss happening before the fluid change?

deanshark

Correct logic but your link is the wrong sensor. that's for the temp gauge. The one that sends signal to the ECU is:
https://www.lexuspartsnow.com/parts/..._system,,89422
FWIW, I got mine for my 93 from Autozone for about $20 and never had a problem with it. Even after taking it from my 93 and putting in my 94 a few years ago.
He changed the fluid because of the miss.
I agree with bradland on an ignition component but I would think maybe a coil is starting to fail. The drivers side coil gets pretty hot and is known to fail first.

bradland

The friction modifiers designed for Toyota (Aisin) transmissions are simply not the same as those designed for a Honda trans or a Nissan trans. Or any other brand, import OR domestic, for that matter.
Using these multi-vehicle products equates to a steady diet of Churches chicken and fries. You are slowly killing it! This video has been posted several times before. It's about 40 min long so at the very least watch the last 6 or 7 minutes, you'll be glad you did.

Sin1UZFE

So essentially one will not notice a difference in performance until it eventually dies?