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This infomation comes from another thread, but I thought it needed it's own post to make people aware.

ANDI 09-17-2001 02:37 PM
"Do you have an EGT gauge?

When I tried an OBD1 ECU in my '98TT, my car ran much leaner as if it were staying in closed loop (13:1 on the wideband o2, on boost, like Scott Smith's car). EGT's went from ~650C at the top of the DP on a long pull to ~710C at the top of the DP on a long pull.... not good.. I couldn't change the AF at all even by modifying the MAF signal w/ various fuel computers.

Some '98s seem to do this, others don't... that's why I'm asking...

Andi"

ANDI 09-19-2001 01:56 AM
"Back when I had an OBD1 ECU, I could easily see 700C by the end of a 40 to 120 pull. The wideband o2 said I was running 13:1. Not cool. FYI I tried 4 different combinations with the same results: MAF with no fuel computer, S-AFC with MAF, VPC by itself, and VPC with a SFC-R. None of these combinations changed the results at all; it was as if the OBD1 computer was ignoring the MAF reading and staying in closed loop! Weird shit.... This was with a stock '94 ECU. Same results when I tried it with a different OBD1 ECU, this one being a EuroX modified ECU.

Then I put the '98 ECU back in, didn't change anythign else, and EGTs were back to maxing out at 650-660C at the end of a long pull.

I guess my '98 is one of the few that really just doesn't like OBD1 ECU's.... and my guess is that Scott Smith's '98 is another...

Andi"

GRANT 09-19-2001 10:57 AM
"1200F sounds high to me. Thats 649C. The highest I see at the end of the 1/4 is 600C on my GReddy gauge. And that was with the car a leaned out a tad too much and some timing retard (About 1 degree). I'd shoot for 580-590 at the end of the 1/4, but tuning via EGTs is probably a very bad idea.

Of course on the street, with the probe in the DP heat soak plays a huge roll. I'll see 640C at the end of 4th if the car had been left at a high RPM prior to the run. IMO the only use of an EGT gauge is to tell you if something is wrong, you can't really tune by it."

ANDI 09-19-2001 12:06 PM

"Grant,

FYI my best runs in the 1/4 (121mph) were with a peak EGT of 620C.

And trying to find out if something is wrong by using EGT is exactly what we're doing here.... not tuning, just seeing if the car likes the ECU...

Andi"

STEVE JARVIS 01-29-2002 03:38 PM
"Andi,

I seem to be having the same problem as you did.

"When I tried an OBD1 ECU in my '98TT, my car ran much leaner as if it were staying in closed loop (13:1 on the wideband o2, on boost, like Scott Smith's car). EGT's went from ~650C at the top of the DP on a long pull to ~710C at the top of the DP on a long pull.... not good.. I couldn't change the AF at all even by modifying the MAF signal w/ various fuel computers. "

I have tried two different S-AFC's and the best I can tell they are both working. I am going to change my ECU back and see if my EGT's go down. At 16.5psi I see about 650C. At 18.5psi I see about 680C. At 20.5psi I see 720C all at the top of 3rd. I am going to switch the ECU's tonight, I will let you know what I find. I currently have a 95 ECU so it may not be a 98 model specific problem.

We will see how many lights and codes appear when you run an OBDII with a single.

Later, Steve"

STEVE JARVIS 01-30-2002 08:27 AM
"The initial test show that the ECU swap cured my lean condition. I typically see 660-680C EGT's at the top of 3rd gear when running 18.5psi. After doing the ECU swap my EGT's were 600C to 620C. I still had my S-AFC adding about 25% additional fuel. I am going to reduce my S-AFC settings at lunch and see if the EGT's go back to about 650C. If this is the case, I will then go to 20.5psi. Usually my EGT's are 720C at the top of 3rd. I will see if they are still in the 650C range. If this is the case I will ask Reg Reimer (who has been looking at the base Toyota programs) why this may be happening to cars going from OBDI to OBDII ECU's.

BTW I now have a check engine light and a big ! in the middle of my tach. I guess I just have a couple more light bulbs to pull.

Later, Steve"

ANDI 01-30-2002 10:34 AM
"Yep, it definitely sounds like you're having the same problem I did. What year is your car again?

For the check engine light, Dennis Walsh says you can put 1k ohm 1/4 watt resistors in place of all the VSVs you removed w/ the single....

Andi"

STEVE JARVIS 01-30-2002 04:39 PM
"Andi,

I have a 1997.

Thanks for the resistor info.

Zerosoul,

The one time I went all the way into fifth (a little over 140mph) at 18.5psi my EGT's were at 720C by the end of the run. This is with 680C at the top of third. I would assume that 650C at the top of third would probably be about 690C on at the top of fifth.

Later, Steve"

STEVE JARVIS 01-31-2002 09:41 AM
"The ECU has fixed my problem.

I did a couple of pulls this morning with my S-AFC at +5% and my EGT's were 645C at the top of 3rd. So here is the comparision.

95 ECU, 18.5psi, measured at the top of 3rd, S-AFC set from 0 to +50% (It didn't make any difference), EGT's 680C
97 ECU, 18.5psi, measured at the top of 3rd. S-AFC set at +25%, EGT's 615C
97 ECU, 18.5psi, measured at the top of 3rd. S-AFC set at +5%, EGT's 645C

I also did a pull to redline in 3rd and 4th and my EGT's were 675C at the top of 4th. Previously with the 95 ECU my EGT's at the top of 4th were 720C. I don't think I will see a lot of change in my EGT's from the top of 4th to the top of 5th. I guess I will see on the roadcourse, but I know I can add more fuel now. I guess I need to get back to the dyno and tune the thing now. I guess I could push it to 550rwhp if I wanted to push the injectors to about 95%.

Later, Steve"

STEVE JARVIS 02-01-2002 10:47 AM
"I did a third gear pull at 20psi and no more increasing EGT's as the boost goes up.

In comparison to the above post.

97 ECU, 20psi, measured at the top of 3rd. S-AFC set at +5%, EGT's 645C

I wonder if G-Force changes anything about the fuel curve when they do an increases redline ECU? I find it hard to believe that this would only be a problem on a couple of cars.

BTW I am going to post this info. to a new post. I think it is relavant and some people might not have read this long post.

Later, Steve"

Hope this helps someone, Steve
 

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Discussion Starter #2
More info.

ANDI 02-01-2002 01:41 PM
"Steve,

Remember that I had this problem with a NON-modified ECU. A bone stock '94TT 6sp ECU....

Andi"

STEVE JARVIS 02-01-2002 03:29 PM
"Mine was also a bone stock 95 ECU.

Later, Steve"
 

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high egt's too

hello..chalk up another 98 tt owner who had problems with high EGT's when I swapped out my 98 ecu for a 94 ecu.
I bought a 94 ECU from Jarret at POWERHOUSE RACING...he told me that he had never heard anything bad about putting an obd-1 ecu in a 98 tt.......well Jarret needs to get off his butt and do some research.
I see 750+ EGT when I run my car moderately hard..say up to 110 mph...and these EGT's are always consistant.
When I had my on the dyno, the EGT's shot up to over 900...OUCH...needles to say I went back to my 98 ECU.
 

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Now the question is what causes this. There must be a difference is some of the sensor voltages btw 98 and previous year cars. Different voltage = different fuel injector pulses or is it that the injector drivers in the ECU have changed and therefore they changed the injectors in 98's? I'm not talking about some drastic change, but something very minor, like different manufacturer for sensors or injectors.
The second question is why do aftermarket fuel controllers not have an effect on fuel trim with the pre 98 ECU's in a 98, but they work fine on 98 ECU's in 98's?
 

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SHIT, well this sucks, seeing as i just put a modified (8500 rpm redline) ECU from PHR into my car. I am still on stock twins but am going single over the next few weeks. I am worried that it'll run too lean under boost. I guess i'll just have to stay out of boost for now....
:(
Peter
 

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We need some resolution to this issue, plus I'm curious as hell. :D
 

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I've done some more research on this and have found that the cases where the obd1 does not work are rare. I am praying that mine will be OK. From what i gather, for every bad swap, there are 20 good swaps. I would like to know why though...
Peter
 

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Was just reviewing this old post. My guess on this matter would be that the OBD1 ECU's are not mapped up to as high a MAF voltage as OBD2 ECU's. In other words once the MAF hits a certain voltage it reaches the top cell in the fuel table and it will only use the injector fireing pulse width as specified in that top row and thats it. Anymore input voltage will not increase the fuel supply (canceling out any piggyback). The top cell pulse width may only take advantage of 80-90% of the capacity of the fuel system. Someone who hacks the HEX codes in the ecu's will know this for certain. But I think its the only thing that makese sense. The ECU can easily be determined if its in closed loop mode if the O2 voltage off OX1 is cycling up and down or if its hanging out in the top range say .80V+ unless your dead lean. I doubt the ECU is actually in closed loop mode as if that were the case the voltage of OX1 will actually be cycling from 0-1v.

My guess would be the guys who are actually having lean problems are actually running near the edge of their fuel system capacity and the extra injector pulse width of the OBDII ecu gives them some more fuel. Problem can probably be fixed by raising higher base fuel pressure or bigger injectors so that you don't have to run so much injector pulse width to get the amount of fuel you want.
 

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Kal said:
Was just reviewing this old post. My guess on this matter would be that the OBD1 ECU's are not mapped up to as high a MAF voltage as OBD2 ECU's. In other words once the MAF hits a certain voltage it reaches the top cell in the fuel table and it will only use the injector fireing pulse width as specified in that top row and thats it. Anymore input voltage will not increase the fuel supply (canceling out any piggyback). The top cell pulse width may only take advantage of 80-90% of the capacity of the fuel system. Someone who hacks the HEX codes in the ecu's will know this for certain. But I think its the only thing that makese sense. The ECU can easily be determined if its in closed loop mode if the O2 voltage off OX1 is cycling up and down or if its hanging out in the top range say .80V+ unless your dead lean. I doubt the ECU is actually in closed loop mode as if that were the case the voltage of OX1 will actually be cycling from 0-1v.

My guess would be the guys who are actually having lean problems are actually running near the edge of their fuel system capacity and the extra injector pulse width of the OBDII ecu gives them some more fuel. Problem can probably be fixed by raising higher base fuel pressure or bigger injectors so that you don't have to run so much injector pulse width to get the amount of fuel you want.
Maybe you're right -- that certainly would explain why I had the issue when at BPU++++ but it went away once I went single/bigfuel system. (I.e. once I was single/bigfuelsystem, I was able to run a 94 ECU without problems).

Either your theory is correct (i.e. once you're no longer near the ragged edge of the hex map, it's okay to goto a OBD1 ECU in an OBD2 car), or some other sensor or actuator/VSV that gets removed when doing replacing the fuel system is different in OBD1 vs OBD2 cars (perhaps the way the ECUs control the Fuel Pressure Regulator VSV?).

One thing to investigate is whether fuel pressure is still rising 1:1 with boost with an OBD1 ECU like it is with an OBD2 ECU. The reason I say this is because of the fuel pressure VSV -- it disconnects the fuel pressure regulator from the manifold for hot starts -- and MAYBE the OBD1 ECU controls the fuel pressure regulator VSV differently than the OBD2 ECU and is disconnecting your fuel pressure regulator and taking away your 1:1 rise of fuel pressure with boost.

Just a thought.......

One of these two theories is right.

Andi
 
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