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Another MAF issue, I've searched, I'm stumped

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Old 11-28-13, 08:46 AM   #1
MongooseGA
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Default Another MAF issue, I've searched, I'm stumped

Hey all,

I've been pouring over threads and PMing some members with knowledge, but it seems everyone is stumped. I'll keep this short.

With my MAF plugged in, my 1995 SC400 runs terribly in the low-range. Hesitation, sputtering, and what sounds/feels like a misfire. It's pretty rough. This stops and the engine pulls smoothly after about 2-2,300 RPM. The car shows no CEL whatsoever with the MAF plugged in.

With the MAF unplugged, the engine runs beautifully in all RPM ranges. Obviously the TRAC and CEL are on and the car gets crap mileage but otherwise it's just the way it should be.

I have replaced my MAF with one from aliga with no luck, swapped in another known good unit from a neighbor with no luck, and have reset the ECU with no luck.

When I go to check codes by jumping the Te1 and E1 ports, here's what I get:

http://smg.photobucket.com/user/Mong...44614.mp4.html

O/D Light and CEL flash intermittently, which tells me no codes.

The car has new plugs, wires, coils, oils, filters. BFI mod, K&N, nothing else touched in the intake systems that I can think of.


Thoughts?
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Old 11-28-13, 08:58 AM   #2
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Possibly your ecu is going bad. That's first thing I'd look at.
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Old 11-28-13, 09:30 AM   #3
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Quote:
Originally Posted by Vrank View Post
Possibly your ecu is going bad. That's first thing I'd look at.
Any specific ways to test this before I plunk down for a new ECU?
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Old 11-29-13, 10:30 AM   #4
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Something else that crossed my mind, your TPS is calibrated to run without an MAF...something to try, plug the MAF in and calibrate the TPS. If you're gonna go exact, use an ohmmeter and feeler gauge to get it as smooth as possible.
ECU is also something to suspect. GL
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Old 11-29-13, 02:20 PM   #5
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yeah try setting the tps again with the maf in, move the tps around and see if there is a part where you can get it to smooth out. for the ecu you just have to open it up and have a look around, yours isn't showing the symptoms of a completely bad ecu, but it could be in the early stages.
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Old 11-29-13, 06:39 PM   #6
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FWIW, I have never touched my TPS adjustment.

The idle is smooth with the MAF plugged in. When adjusting the TPS, will it basically be a trial and error type of deal when I have to turn the sensor a tiny bit and then drive the car after each increment?

IIRC, the SC400 has 2 TPS', right next to each other. Do they both need to be adjusted or just one of them?
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Old 11-29-13, 06:45 PM   #7
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Quote:
Originally Posted by MongooseGA View Post
FWIW, I have never touched my TPS adjustment.

The idle is smooth with the MAF plugged in. When adjusting the TPS, will it basically be a trial and error type of deal when I have to turn the sensor a tiny bit and then drive the car after each increment?
So when driving, only driving, with the MAF plugged in, the car drives like trash? Idle is smooth? I vote ECU. If you want to confirm the ECU being at fault, remove the ECU, remove the philips screws that hold the cover on, and see for yourself. If you find any leaking capacitors, or burnt areas, ECU is most likely the issue.

Quote:
Originally Posted by MongooseGA View Post
IIRC, the SC400 has 2 TPS', right next to each other. Do they both need to be adjusted or just one of them?
You must have the model that has Trac system. One TPS is for Trac, the other is the main. Whichever one is closest to the Trac cover is not the one you wanna adjust.
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Old 11-29-13, 06:51 PM   #8
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Quote:
Originally Posted by SC400slide View Post
So when driving, only driving, with the MAF plugged in, the car drives like trash? Idle is smooth? I vote ECU. If you want to confirm the ECU being at fault, remove the ECU, remove the philips screws that hold the cover on, and see for yourself. If you find any leaking capacitors, or burnt areas, ECU is most likely the issue.


You must have the model that has Trac system. One TPS is for Trac, the other is the main. Whichever one is closest to the Trac cover is not the one you wanna adjust.
Correct, only driving does the car run poorly. Idle is smooth with the MAF plugged in.

In upper RPMs the engine sounds and feels fine. There is a slight bit of hesitation or 'hiccuping' when accelerating, but the majority of the problem is from a stop to around 2k RPM or so.

My car does have TC, so I'm assuming the TPS we'd be discussing is closer to the intake plenum as the TRAC cover is on the far left (passenger) side of the engine bay.
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Old 11-29-13, 07:40 PM   #9
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Quote:
Originally Posted by MongooseGA View Post
Correct, only driving does the car run poorly. Idle is smooth with the MAF plugged in.

In upper RPMs the engine sounds and feels fine. There is a slight bit of hesitation or 'hiccuping' when accelerating, but the majority of the problem is from a stop to around 2k RPM or so.

My car does have TC, so I'm assuming the TPS we'd be discussing is closer to the intake plenum as the TRAC cover is on the far left (passenger) side of the engine bay.
Click the image to open in full size.

Since I'm too lazy to read through the thread and your list of issues, have you replaced your o2 sensors?

I highly suspect the ECU.
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Old 11-30-13, 11:50 AM   #10
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I have not replaced any o2 sensors.

If the ECU was going bad, would it still allow normal engine performance like I have with my MAF unplugged?
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Old 01-23-14, 03:51 PM   #11
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Update:

I had the ECU completely rebuilt by a tech locally. After reinstalling it in the car, the same problem still persists.

I'm at a loss.

All signs point to MAF, and it's not the MAF. Fixed the ECU, and it wasn't the ECU.

02 Sensors?

When I replaced the MAF, I only replaced the sensor itself. Would there be something in the housing causing these kind of issues?
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Old 02-20-14, 05:08 PM   #12
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Just to update.

Since having the ECU rebuilt and the MAF swapped out, the car has gotten better but still has the same issue.

The tricky thing is, it's not consistent. I've noticed it only sputters maybe 60% of the time until I get to around 2,200 RPM. It accelerates fine for the most part when the engine is cold. Once it warms up is when I notice the loss of power and rough acceleration.

I've also noticed that if I really slowly ease on the the gas is will not sputter. If I give it a moderate amount of throttle, then it will show the issue. I've thought maybe TPS, but wouldn't a bad or misaligned TPS cause a rough idle? My car idles perfectly.

I'll likely swap into new O2 sensors as well. It's about time anyway with 103k.

Thoughts? Questions, comments, concerns?
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Old 02-26-14, 04:35 PM   #13
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Bump. No ideas?
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Old 02-26-14, 07:55 PM   #14
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Just a simple question did you remove the honey comb from the intake?
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Old 02-26-14, 09:29 PM   #15
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No I have not.
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Old 02-26-14, 09:29 PM
 
 
 
 
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