mb2sc400

Hey folks,

Been going through a painful process to try and get an engine surging problem under control. To make matters worse, the transmission finally gave up the ghost in my truck and this has increased the pressure on getting my sc400 back on the road asap. If anyone wants to chime in, please feel free to do so.

Vehicle:

1995 (94 production) Lexus Sc400, stock engine/trans

Problem:

Idle surging from 1100 - 2500 rpm approximately once per second. Can be placed in gear and will drive. Idle smooths out but is crazy high. Allowing the vehicle to idle in drive without any pedal will cause the vehicle to reach 35mph+. Throttle response is crisp in park/neutral despite the surging Disconnecting the MAF produces low, stable idle (about 600 rpm) and delayed / overcalculated throttle response.

Investigated:

Checking for codes while MAF is connected produces nothing but clear status. With it disconnected it obviously is giving out a 24 and nothing else.

Tried a friend's MAF out of a '92 LS400 that I know is functional and it produced the same results.

Removed plenum, upper and lower manifolds and re-installed with gaskets to ensure everything was done properly. Did this because last winter myself and a friend went ahead and changed out the starter. Also wanted to check for any possible vacuum leaks. Went ahead and played with some carb cleaner to try and diagnose any leak I wasn't hearing and could not find anything.

Removed IACV and gave it a good cleaning, reinstalled (another gasket ) .

Checked connectors on IACV and MAF to ensure there was no breakage/shorts.

Checked resistances and voltages on wiring to MAF .

Ordered and tried a new TPS, despite the throttle response I was seeing.

Ordered replacement IACV and ECU. Pulled out my ECU and partially opened it to find the previous owner had an ECU replacement/rebuild already done. the company had soldered the tops of the screws that hold the two PCB's closed, making it impossible for me to check the status of my capacitors.


Currently I'm leaning towards the ECU potentially being the culprit, but again anything you folks can think of for me to check/diagnose, I'd appreciate it. Thanks!

t2d2

iTrader: (8)

If I remember right, one side of my ECU had the screws soldered on but it hadn't been serviced.

mb2sc400

Thanks for the quick reply, and the tip! I opened up the brain again (right side this time!) and was able to examine the goodies. Unfortunately, I was unable to find any signs of concern. Things checked were

trace lines
capacitors, for bulges or leaking
burn marks on the pcb
lifted solder pads, overloaded solder pads
dirt, dust, grime

Overall it appears to be in very good shape. PCB is shiny and new looking and all components look as though they could be sitting on the latest video card board. It does look like the ecu had been serviced at some prior point in its life. Had an autotechmicro.com sticker covering the access point . Coming up with nothing on the webz for that domain but it appeared to have been involved in auto electronics at some point in time.

freakness

Dude,


Try a different, used if necessary, IACV. Check all hoses for vacuum leaks.




.

mb2sc400

Just wanted to give an update to the problems I was having.

Yesterday I managed to get my hands on a 92 sc400 for about $200.00 . It appeared complete and anyway, it had a usable IACV which is what I wanted. After doing a second vacuum leak check, I went ahead and installed the then-removed and cleaned IACV from the "parts" car. Started right up, decent idle. After a minute, idle was perfect and car was running as it should. Further inspection of the old IACV confirms it stuck in the open position.

Thanks for the replies folks, you pointed me in the right direction! Mods feel free to close thread

t2d2

iTrader: (8)

Another win for IACV servicing!