Mrmerlin

I have a 1987 928 S4 Auto.
I swapped in a 1989 GT engine along with its harness and the GT computers.
This was a small conflict between the newer GT harness and the older S4 CE panel, also the diagnostic connection wasnt carried over to the S4.

After getting the engine running it idles very low and I dont know how to raise the idle.
It appears to have had an intake refresh so everything looks new.
Also a fresh MAF was installed.

I Checked the idle stop and it appeared to have been reset to close the throttle plate,
so I reset it to be open about 1 turn more this looked like the original position this also required the TPS to be readjusted and that has been done.

Now with the battery disconnected/ reconnected the idle is still at about 500 RPM cold
after the engine warms up the idle raises to about 650.

The warmup procedure is to hold the pedal open to keep the idle above 800 so the engine will warm up,
once this is done then the engine will idle at 650 and this is too low for the auto trans.
This takes about 4 to 6 mins to warm the engine

I would really like to figure this issue out as the owner isnt really happy with the engine swap.

I talked to the PO of the GT and he said that the engine always idled low. ever since he owned it.
So what does anyone think on how to proceed.

blau928

Hope this helps as I am in the process of calibrating an aftermarket ECU with an auto,

- The idle speed drop signal may be engaged all the time instead of only in D,R,3,2..

- Have you checked the pinouts (30 on the LH for the speed drop for the auto)

- Pin 33 for the IAC valve?

- Pin 8 & 18 on the EZK for the throttle valve switch for idle speed? (corrosion, bad contact, good signal to EZK etc..)

Wouldn't you have to use an S4 EZK instead and raise the idle to 770 +/- 25 with a Sharktuner for the GT cam? Or do all EZK's have the logic to operate an auto if the pins are receiving signals fromt he transmission? (I'm not sure about the last part...)

Best Regards,

PorKen

Wrong chip in the LH?

Correct LH number for GT?

GT harness will have the in-gear input to the LH wired to ground.

(S4-up manuals are all like this. There is no coding for auto/manual for the LH.)

Lizard928

Wrong coding plug, and chip.

GregBBRD

Sharktune. John Speake's newest idle control program is absolutely "the bomb".

You can make a 6.5 liter engine, with nasty cams and all the cool stuff to go with them, idle and warm-up like a stocker.

And with Jim Corenman's Sharkplotter, your pet dog can tune the thing.

Genius, these two guys are.

Mrmerlin

I swapped in the 87 S4 computers and no change of idle.
AFAIK the GT computers are correct for the GT and they were straight swapped over along with the GT harness
Colin what exactly has to happen with the coding plug?
What chip has to swapped? to what chip?

Ken please explain the wire that is grounded and should it be ungrounded , not sure of this info?

Greg the engine harness has been sort of split and I dont have the GT harness diagnostic connection that was original ,
as i didnt swap over the CE panel but used the 87 S4 Ce panel.

Lizard928

Stan,

The coding plug is in the factory manual. If the car is an auto it will be forced to idle at 625rpm vs 775 GT idle.

GregBBRD

There's an adaptor to Sharktune pre-diagnostic connector vehicles. Colin's idea should raise the idle. Tell the brain that the vehicle is a GT. I'm just not sure if that will affect the pressure reduction when the transmission shifts.....once the brain thinks there is a manual transmission attached to it.

PorKen

Pin 30 at the LH, the idle speed drop input.

Manual wire harnesses have this input go to ground. I could not find where.

I use this input for a clutch switch. I had to substitute terminal 30 with my own terminal/wire.

The LH does NOT have any wiring to the coding plug automatic pin. Only the EZ uses it.

The chip for autos and manuals is the same - the manual LH code thinks the transmission is always in neutral.

Mrmerlin

Thanks for your assistance in working through this with me.

Here are pictures of the code plug its from the GT setup ,

NOTE would moving the center lower pin one slot to the left in the picture work ,
as this would be for the auto version for 91 RON as seen in the last plug layout in the first picture.
Will this raise the starting idle? swapping the pin? to the lats config? on the first picture?


These are the computer #s I have,
IDK if they are the correct PN for this GT engine.

the LH appears to have been opened,
I didnt investigate chips.

Ken are you suggesting that I remove the connection to ground for pin 30?

the main issue is the very low idle at start up, now that I see the LH has been opened,
I wonder what else may be messed with inside the computer.

When the engine is running off of idle it runs very strong and pulls hard

PorKen

No, at least for idle speed.

Coding pin 3, automatic trans, is ONLY wired to the EZ.


The only way the LH knows it is in a manual transmission car is if the input to pin 30 never changes.


See the WSM wiring diagram for the LH/EZ, at C-87.
From CE plug U terminal 23 there is a wire marked \NUR M481/ = only manual trans, which goes to ground MPVI.
U23 is also wired to the starter relay ground.

PorKen

LH part and EPROM number ID thread - https://rennlist.com/forums/928-foru...-and-year.html

The LH is from a 89GT, but the EPROM?

Mrmerlin

Here is a picture of the LH it has a 928 S4 race chip in it who makes this ?

PorKen

Probably an (eBay) AutoThority copy.

John Speake

Hi Stan
When the idle system works correctly, the idle speed will be higher as engine warms up, settling back to 775 for GT ECUs or 675 for Autos. So there's a problem somewhere

The problem for an auto is that there is no idle speed drop when engaging a gear with GT ECUs, whatever the code plug setting.

However if you have access to a Sharktuner 2 there's a new idle control system that is fully programmable to get exactly what you need. Jim can supply.

But first you need to get to the bottom of the very low initial idle speed, maybe the ISV is stuck ?