here's my car
94 sc300 NA-T
small-ish te04 turbo with V8 maf, AFC, Walbro, 440 injectors, wideband. My bov is recirculated and my maf is before the turbo, not inline before the TB.

I love the car when it's running fine, but I'm having some weird issues when it's hot (after an ~hr of of driving)
When the car is cold it's fine starts fine drives fine, idles fine.
Also when I left off the gas abruptly, there is a chance the rpm neddle can just drop, and the car stalls. I've had to crank up my idle to 1,200rpm to keep it from dropping too far. this has worked fine in the past, but now it's not even enough. I may have to adjust it even higher like 1800 rpm


When when it gets hot, the car can idle erratically. It was so bad, it wasn't drivable, it would just keep stalling out. I've been able to alleviate this, but turning my TPS all the way counter-clockwise. and playing with the idle screw by turning it in clockwise. This moves the stud further down which opens the throttle plate more.
So the car is more or less drivable, but it still seems to stumble a bit. Even sitting at idle and not touching the pedal, I can see the rpm needle drop 500 rpm's and bounce back up. this happens every couple of minutes.

things I've already done to try and fix this:
Fuel ECU bypass, did nothing so I reverted back
changed fuel filter, throttle body this also includes a different set of TPS and IAC.
checked for vacuum leaks
changed O2 sensor in DP (this is now wired to ecu pins 47 and 48)

I'm going to change the fuel pump next.
Not sure what else I can do, maybe injectors?

What would make it so that I have to trun my TPS all the way counter clockwise??? It used to sit in the middle with equal amounts of room to the left or right.....

and the thing is I have no CEL codes....
Why would it only act up when HOT? (I hear this can happen with the fuel pump so, I'm trying that next)

any suggestions?

thanks guys

 

try cleaning maf and TB, and double check for vac leaks by using brake clean or carb cleaner

 

the maf can be cleaned? I thought spraying anything on that element could damage it, no?

I'm on my 2nd TB right now, I swapped it out cause I thought either the IAC or TPs could be bad. But it didn't help, still have the same symptoms.
I tried spraying TB cleaner in both, with no luck

 

you should perform a fuel pressure test before replacing the fuel pump, no sense in waisting money. how long has the car been doing this. make sure the MAF sensor isn't damaged, look really close because sometimes it looks like the wire is intact but is broken at one of the ends. some stores have MAF cleaning spray is in fact the sensor is dirty

 

Did you do the TPS adjustment.

Ian

 

thanks for the input guys:

oh forgot to add,
I tried another maf before, and it made no difference
(unless the other maf was a bit flaky too? )

and yeah I did the adjustment my self

 

If the V8 MAF you have is a karmen vortex, DO NOT SPRAY ANYTHING INTO IT.

I've been through two of the karmen vortex MAF's on my car (currently not using one, running speed density) and they don't really seem to like boost. The sealer on the two that I have used would fail on the top cover creating a boost / vac leak.

KC

 

what's puzzling is that the car only acts up when hot.....

 

Any black smoke out the back? Do you know if it's running rich, or lean?

Couple things I'd look into...

Coolant temp sensor. A bad coolant temp sensor can cause a surprising number of weird drivability issues and it's often overlooked. If you have an ohm meter, you could check it's resistance when the car is cold, warm the car up and check it again to see how much it changes.

IAT sensor. I don't think this would cause the problem your having but anything is possible, and its reading depends on temps.

Sticking EGR. Doesn't really make sense that it would only cause problems when hot, but who knows.

Even if your engine light is off, you should still check for codes.

There's many other things it could be, I'd start by checking the coolant temp sensor. If it is stuck and reading low, the engine will run rich.

KC

 

No black smoke, I'm reading 14.7 AFR at idle on my wideband.

ah! coolent sensor... never considered looking into that,
you might be on to something
I only other thing I found that could be temperature related was Walbro fuel pumps going bad. And acting erratic when hot (well very warm anyways after it's been running for a while)
But I'll definitely look into the sensor
btw: the temp gauge on my dash is still acting normal, would a bad temp sensor still show that?

thanks!

 

You probably have at least two sensors (not a 300 expert), mine has three. There should be at least one for the ECU and a seperate one for the gauge. If your AFR is ok then it's probably not the problem, but still worth a check. The coolant temp sensor is a major input to the ECU as far as AFR is concerned.

Does the problem eventually go away, or does it persist until the car cools down?

KC

 

if the car sits overnight, it starts up and drives perfect with no hiccups.

If I restart within a few hrs it can still act flakey

I looked over my manual
the engine coolent temp prob failure should kick out
a code 22. I have no cel, (last cel was the pre-cat O2 sensor, but I've wired on in, and it went away; this was 2 months ago)
but I guess it doesn't hurt to swap it as I'm still on the original one, and I'm at 74k miles

I'm gonna pressure test it this weekend, and swap the fuel pump this weekend. I already have a denso one on hand already and I've never been crazy about the Walbro, it's pretty loud.
I'll try the temp sensor too if I have enough time (if not I'll do it the following weekend)

 

came up with 2 more possibilities.....

could this be an issue with closed loop / open loop ecu settings?

also if my timing if off by a tooth or two on the passenger side cam, would kind of problems would I see?

thanks

 

Ok, rethinking this. If your AFR is good, allot of things can be, in theory, eliminated.

What about you idle air contorl motor?

Does the car run fine off idle, even when it is acting up?

You say you've adjusted your idle screw, does the idle screw act on the throttle body, opening the throttle plate? Opening the throttle plate would let air in past the idle air control motor.

KC

 

thanks for the continued input

Well the thing is, I'm on my 2nd throttle body right now.
I originally though there was a problem with the idle air control, so I swapped out the TB with another one (which had it's own IAC and TPS sensors)

I'm gonna try switching on the "test" mode of the ecu
(it's supposed to make it more sensitive to problems)

But right now the top 2 suspects are:
fuel pump and timing being off (my passenger side cam is off by like 1 or 2 teeth, I'll need to take a pic to confirm)