AO

Thursday I went to the Waterford Hills for a track day. Had a blast. I made a point to really try and feel the car for what I perceived as a hole in the acceleration curve. It always seems like there was a dead spot at about 5500 RPM.

The car has Ken's S300 4.0 chips installed.

Yesterday, I hooked up my WBO2 to the car and drove around. It was way lean. running mostly in the 15's and 16's and sure enough right around 5500RPM it went to 17.1. No wonder it feels down on power.

I investigated further by putting a fuel rail pressure gauge on and the FP at idle is only 30PSI. So I'm assuming it dod not get the S4 FPR like it's supposed to have.

I clearly don't want to scorch a valve or burn a piston, so I decided the safe thing to do would be to install the stock chips (or in my case, the spare set of brains with stock chips). Car fired up fine and ran well.

But here's the weird thing... with the stock chips, the car ran even leaner! ???

The 30PSI at idle seems low, doesn't it? I'm wondering if I have a weak fuel pump.

I thought I had a FPR from my GT, but I don't so I'll need to get one, but I just thought this was weird and would report to see if anyone had any super thoughts on what could be causing it to go so lean.

Cosmo Kramer

If I remember correctly I think that's what my 85 used to have for fuel pressure at idle. Jump the fuel pump relay with the car shut off and see what you get. If it has the s3 regulator I believe it should be 38 psi. Maybe its only delivering 30 psi all the time and that would certainly be the cause for a lean condition.

AO

Does this look right? This looks like an adjustable FPR...

Cosmo Kramer

That looks like the adjustable one the 928 vendors used to sell years ago, I had one in my fast supercharger kit and sold it. You need to check the fuel pressure at 0 vacuum to make sure it is raising correctly.

AO

Thanks Ted. Just made some adjustments. Taking it out for a spin to see f this helps.

jcorenman

Andrew,

Run the WSM flow test. A weak pump may be able to put out 38 or 55 psi and satisfy the pressure gauge, but might not be able to provide enough flow at that pressure to feed the engine. If the pump can't keep up with the engine then the pressure drops and it goes lean-- under load at higher RPMs of course.

Disconnect the return line at the tank, extend it outside the car and into a jug, and measure flow: should be 2.5L/min (i.e. 1.25L in a 30 second period, or 1Kg if you have a suitable scale). That's enough to keep a set of 24# injectors full fed with only a bit of headroom.

Cheers, Jim

AO

Thanks Jim, but in this case, I just cranked the screw (bolt) down to increase the FP at idle. Got it up to ~48psi. Took it out for a full pull and it is now pretty much dead nuts on. Still a tad lean at 6k (15.1) but the came down nicely to 14.3 at redline.

The car has never felt this strong! Woo Hoo. Need to do another track day!

Btw, I noticed this was disconnected. What is this? It's next to the Temp II.

69gaugeman

Looks like knock sensor, Andrew

AO

But 86 doesn't use them. And it looks factory.

Cosmo Kramer

The S3 doesn't have a knock sensor. I can't remember what that one is for.

Landseer

That connects to the tps.

PorKen

Fuel pressure at idle with the vacuum hose connected for a S4 FPR (which the S300s chips expect) should be around 41psi (55 - 14.5).
WOT should be in the mid to high 12's depending on the weather and what gear you are in.


That's the TPS extension connector. Check the WOT input at the LH plug.
If the TPS is original, the WOT part of the TPS probably isn't working.
Pinouts - https://rennlist.com/forums/928-foru...2-pinouts.html


AFTER you have verified a good WOT input, you can also plug the LH harness plugs (brown wires) together.
This will richen WOT somewhat with the S300s chips. (Stock it switches to a different O2 loop map.)


Do not plug the EZF connectors (green wires) together!
This shorts the air temp sensor and significantly reduces performance.
This is only for use with really bad fuel, for example.

PorKen

I'd recommend ditching the adjustable FPR and putting in a '87 (or '88 with GB hose kit).
I always had problems with the adjustable ones plus they didn't seem to have the same fuel curve as the factory ones.

If need be, the S3.S300s chips are programmed for WOT to be separately adjustable electronically with a 1K pot in the WOT input to the LH.


Note you also can do a quick check for WOT anytime at the diagnostic connector.
C = Ground, B = WOT, A = O2 status from LH

Bridge B->C to disable idle control and the O2 loop to set the idle speed and MAF CO.

AO

Thanks Ken. I'm wiped out for today. Been doing a few things on the Honey-Do list. I'll get a 87/88 FPR on order, but at least I found the culprit.

I'm surprised I didn't notice the TPS was disconnected previously. It's connected now. Thanks for the tip on the diagnostic port. These 85/86 machines are still a bit foreign to me, but I'm learning quickly.

BC

You specifically want the early 87 FPR that has a hose barb.