Thought I would make a new thread as I realized my car has two different issues.

In the other thread I was able to diagnose and fix the stuck on injector issue by replacing the ECU. It was dumping a bunch of fuel on cold starts, hesitating, running very rough, etc. Replaced ECU now it runs great.


However, I have another issue that I now realize is separate. Basically once the car is totally warmed up (15-20 minutes) the idle randomly fluctuates. For example it's sitting around 500-600 rpms, then it will randomly dip to 200 then back... or down to 400 and back. There is kind of a "hiccup" from the engine when it does this. It almost stalls but doesn't, and there is no rhythm to how it does it (it's very random). This only appears at idle without my foot on the gas. Car has full power, and doesn't hesitate with throttle.

Here's a short video video clip. It's poor quality but gives a idea to what the tach is doing when this happens. Also I already cleaned and tried another IACV, but that didn't work.

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Also I noticed today a light ticking coming from underneath the car mainly on the driver's side.

Possibly an exhaust leak causing false readings at the o2 sensor?

I guess that would make sense because the issue happens once the car warms up and goes into closed loop, so up until then o2 sensors aren't being used.

Any other ideas appreciated, thanks.

 

tried cleaning maf? any check engine lights? may be a bad connection from coolant temp to ecu

 

No CEL, I still tried to pull codes but came up empty.

Well I was told not to use cleaner on this type of MAF. I tried blowing it off with compress air, and checked
resistance, but didn't do anything.

I actually replaced the ECT sensor as well.

 

It's definitely only a warm idle issue. The car runs awesome. Today driving around I made pretty good time, didn't have to sit at traffic lights for very long, etc so it didn't act up.

Once I got home I let it idle for I'd say 40 seconds then i felt a couple misses and the idle dropped and stabilized again. It's weird it only does it when warm and when idling without moving for an extended amount of time.

 

Hey I had this very similar issue and found the problem to be a clogged egr valve and took it our and cleaned the whole egr system and the idle problem almost went away. my motor is a na-t sc300 92 runs much better and doesnt have the hesitations and stall issue

 

have you check your spark plug gaps...or could have a lot of carbon buildup from the bad ecu that was dumping fuel..

 

I had the same issues, this is what i did to fix...

Step 1:
Cleaned ground battery terminal, followed ground wire to under battery tray, took that off, sanded and cleaned that. Then I followed the ground a bit more to under the intake manifold. Removed that, sanded and installed that back on. Follow it even more and you'll see another point, remove that, sand, clean, reinstall.

Step 2:
Replaced AFM with a newer one from another club lexus member. You are correct that you shouldn't clean the older style MAFs.

Step 3:
Remove IACV / throttle body, etc and clean the heck out of those with carbon cleaner. Do not clean the sensor connection point with carb cleaner. Reconnect everything.

Step 4:
Reset ECU

My car hasn't stalled in a few months since I did all of this. I'm not saying it's a technical repair, it just worked for me.

 

You blew compressed air through the MAF???? Hope you didnt damage the Hot WIre.

 

Funny you mention that. Today it finally thought a code 71. Which is EGR malfunction. So I guess the best thing to do would be take it off and clean it first? Did you have to drain your coolant in order to do so?


I actually already replaced the plugs. But your right the old ones did look pretty bad.

The maf is a karmen vortex not hotwire.

 

a injector not firing? or stuck?

 

I have this exact issue with my sc400, I was told to look at the vacuum system.. but the egr makes sense.

I have, however, replaced the plugs / wires etc.. and reset the ecu to no avail. Like yours, mine runs great, even after putting over 300k on it.. but just recently starting surging when warm at idle.

If I do find anything new I will post here.

 

Yeah let me know if you figure out anything.


Does anyone know if I have to drain the coolant in order to remove the EGR? I'd rather not if I didn't have to.

 

Haven't cleaned the EGR yet, just been living with the issue for now.

I also noticed a surge when letting off the gas pedal... especially coasting downhill.