tftimm

Summary:

1) Already changed coils (both): no difference
2) Car starts right up, no Check Engine Light
3) After driving for a few minutes, car misfires only when accelarating
4) At cruising speed, car doesn't hesitate, only when accelerating
5) Idle becomes rough when car is warm, at stoplight, etc.

I first read that it might be the Drivers side coil...changed it, no difference. Did the same with passenger coil...no difference.

The car starts right up, no problems, and even on my drive to work this morning, it was fine. I also did NO heavy accelerating, just light. If I step on it, the car hesitates and then seems like it misfires, almost like it needs more gas, or more air. Interestingly enough, it does all this with the Check Engine light off. It has come on periodically since I owned the car 3 years ago, but it has always run smooth.

My guess is it is Fuel Pump related. I would think that if the spark plugs or wires were the problem, the issue would always be there. I read that the Fuel Pump can get worse once the car warms up. Also, the lines would get full after it sit, which is why it starts up and the problems get worse when it warms up.

Now, do I have to take the fuel tank out to get to the pump? I know how to remove the back seats, the trunk liner, etc., but it seems like you can access the cove where the fuel pump is by removing the back seats.

Thoughts? Any help is appreciated.

SC400slide

iTrader: (11)

I know this is mentioned in 99% of the "hesitation threads" but did you try the 12v mod on the Fuel pump ECU?

https://www.clublexus.com/forums/per...tures-faq.html

Also, this thread is very helpful.

https://www.clublexus.com/forums/per...on-issues.html

Ali SC3

iTrader: (10)

12v fuel mod doesn't cause it to run rough, it causes it to not run at all.
I have never heard of a fuel pump causing that issue please don't start removing your fuel pump.

sounds like you have bad plugs, or a worn rotor in the distributors, you should do an ignition tune up.
ignition problems can change under different conditions I wouldn't rule out all the ignition by that logic.
when was the last time the plugs were done, rotors, o2 sensors, how many miles since then and how many miles total on the car.
more information would be helpful, but I think its your ignition probably the rotors or plugs if I had to guess.

tftimm

I can't see some of the pictures in the post, like the ones after the ones with the spliced wires.

Will this cause the car to run any richer?

Ali SC3

iTrader: (10)

I don;t think you need to do the 12v mod. it doesnt run richer but it makes the pump work a bit more and will disable the safety pump shutoff functions. could also be a bad ecu, does your passenger floor ever get damp? may be worth a look inside.

tftimm

Would these things cuase the Check Engine light to turn on?

First off: 1992, 110K, and I haven't done the plubs, wires, etc.

I did recently (this past weekend) change the coils and it wasn't that. I'm leaning towards plugs/wires might be the culprit.

Second: Yes, I did the the problem with the passenger side floor getting damp due to the AC line getting clogged...hasn't happened in months and has been fixed. But with the ECU out, wouldn't that cause the Check Engine and potentially other lights and problems?

I don't want to sink a bunch of money into this, but I do want to get it fixed. Thanks for all your help, the more comments the better.

soarer13oy

iTrader: (13)

MAybe dirty fuel filter? Has it been changed?

tftimm

I haven't changed the fuel filter. That was going to be my next (DIY) shot. I read that the SC hardly ever, if ever, needs the fuel filter cleaned.

I did put a bottle and a half of the Chevron Techron Fuel System Cleaner and it is running a little smoother. I put it in after a fill up, I should have done it before, but I didn't

Is the fuel filter worth changing? I know it is only about $20 but it is a messy job, although I'd probably do it myself.

Ali SC3

iTrader: (10)

no they don't usually cause the check engine light to come on.
you have mileage working in your favor but if it has original stuff on it those need to be changed now. typically they do some stuff with the timing belt but if you put that job off these smaller things go unnoticed.

wires rarely go bad unless they are pinched or bent badly, but what is a maintanance item are the plugs and the rotors inside the distributor. the contact part wears down on the rotor and you start to randomly drop cylinders, until it wears enough that it can leave you misfiring and stranded. o2 sensors you should look to replace soon for mileage reasons but probably not dead enough to cause this problem.

So I think its either the plugs/rotor, or you may have a bad ecu. the bad ecu affects fuelling and causes misfires and does not typically throw codes (fun right). only way on that one for sure is to open it up and have a look, but your SC sounds like its low mileage and could be in nice condition so I would start with the ignition stuff I mentioned first and go from there, but the warming up part and the moisture does make me lean now towards bad ecu, so I will leave it up to you which one you want to do first but plugs and rotors should be done or someday ignition will go out while driving.

I don't think its fuel because the fuel pump ecu and 12V mod usually means the car wont start up again, and if you are misfiring you are likely having an overfuelling issue or ignition issue, ecu can cause random overfuelling usually when car warms up because the capacitors go bad inside it and they affect the injection circuits from what most people who have had bad ecu's describe. bad idle, misfires, and overfuelling are almost the exact symptoms of a bad ecu.

tftimm

When you say ECU, you mean the ECU for the car, or the fuel pump ECU? Is changing the main ECU for the car a big change? Is that the one under the seat? How much for a new one, is it a DIY with the new ECU?

I am going to check on changing the plugs and rotors. It is wierd, it definitely gets worse during strong accleration, would this be typical of the ECU or rotors?

This happened during a longer 80 mile drive...when we arrived, it started to misfire.

SC400slide

iTrader: (11)

The main Engine ECU. Changing it is easy. It's on the floorboard underneath the carpeting and a cover on the passenger side.

tftimm

Should I buy a rebuild or new ECU in case it ends up being the problem

SC400slide

iTrader: (11)

I'd recommend you rebuild it or buy a low(er) miles used one. New is a lot more costly, rebuilding is your best bet. Cost around 100 bucks, Lextech can do it for 80. I'ts possible to do yourself...if you have solder experience.

Ali SC3

iTrader: (10)

yeah fir thing to do is to find it under the carpet and the plastic cover, then unplug it and bring it inside to pop it open for an inspection. if you see leaky capacitors and brown goo over the board then you know its time for some repair.

tftimm

Will do.

Is there a way I can tell if it is the car's ECU or the fuel ECU?

The car may be running a bit rich. I have only started on the tank of fuel so it is difficult to tell. If it is running rich, does this help me narrow down the possibilities?