djessence

Hey Crew,

Need some help diagnosing a surging engine. It's not specifically at idle so I'm not sure if the surging idle threads apply. Details:

- Fresh Rebuild S5 engine (new hard seals, used housings and irons)
- installed in 1st gen racecar - emissions stripped, and all of the non needed harness has been de-pinned
- dual fuel pump and surge tank (LP pump to surge tank, HP denso pump to engine)
- no findable vacuum leaks
- OMP removed (premixing), tucked away and plugged in

On startup it seemed to idle fine. When we flipped the electric fan on, the surging seemed to start. Flip the fan off, and it seemed to be less.

Surges now at idle. Surges at constant 1/4 throttle. Surges at constant 1/2 throttle. Haven't tried to drive it yet. Best way I can describe the surge is its stable for 3-4s then surges up fast for 1 second and then settles back down. We don't have the tach working yet but will have one in on Sunday.

See videos for sounds (youtube links below).

Any help you can provide (or other applicable threads) would be very helpful.

Jesse

KansasCityREPU

iTrader: (9)

The fan might be pulling too much current and robbing a sensor. Maybe a bad ground for the fan.

djessence

Update: Once its warmed up, it doesn't seem to surge at idle. But as soon as you touch the throttle, it starts surging.

TPS?

The little vacuum solenoid/dashpot thing on the wrong of the UIM seems to also surge a little but not too sure what this does. See photo:




On the driver side of the UIM is some type of device tucked underneath. From the vacuum routing diagram it looks like i should connect the 2 ports - is this needed if emissions is stripped? Could this be causing it? See picture below.

This little dash-pot-y thing - do i need? should I remove? I found it in the manual but its not labelle as to what it is or what it does.


Another video:

GSLSEforme

The ignition timing sounds like it's late,have you checked it? Is the TPS hooked up and working properly?The idle is too low,put a timing light on it.If timing is late it will pull down idle speed and exaggerate/aggravate your surging. Correct timing will minimize it,may eliminate it-not sure about that.

KansasCityREPU

iTrader: (9)

The dash pots are the 5th & 6th port actuators. They need an air source from the exhaust to actuate. They work when the RPM gets to 4K.

Rotary Alkymist

I would source another inlet duct. Unless that's insulation tape? If that's duct tape then you're most likely pulling in unmetered air.

djessence

It's just tape on stock equipment.

[QUOTE=djessence;12371017]Update: Once its warmed up, it doesn't seem to surge at idle. But as soon as you touch the throttle, it starts surging.

TPS?

The little vacuum solenoid/dashpot thing on the wrong of the UIM seems to also surge a little but not too sure what this does. See photo:


The little adjustment screw in this photo was screwing me up. Pulled it out (the nut is jammed) and idles/accelerates very well.


Now though I am fighting through high AFR on moderate to WOT acceleration. AFRs in the 15-17 range which seems too high for WOT. The hunt continues

Rotary Alkymist

It's just tape... why is the tape there? Surely it isn't there for the aesthetics.

GSLSEforme

I'm going with its purpose is to reflect exhaust heat away from air intake tube. Hotter air going into engine=less power. Colder more dense air from front of car= more power,more better.

djessence

it's insulation tape. Also, shiny things right?

Since you guys are super helpful in the first gen forum and I assume some others have put a hacked harness S5 engine into their first gen - maybe you guys can help out with a few confirmations:
Again engine has no emissions equipment on it:

1. Best spot for the pressure sensor vacuum hose to go to - right now I have it on one of the lower vacuum ports on the lower intake manifold - is this acceptable? Should i put it on the one right below the fuel injector or one of the ones on the firewall side of the upper intake manifold?
2. Best spot for the vacuum connection for the fuel pressure regulator - also have it on one of the lower ports on the lower intake manifold.
3. Secondary injectors are controlled by both RPM >3800 ANDload (ie increased vacuum) correct?
4. Are there reliable aftermarket options for pressure sensor that will work with stock ECU?
5. Secondary injectors should have 12V with key on - is this 12V across the 2 pins? Or 12V from 1 pin, grounding to engine? As well, in the wiring diagrams it says "Grounding time is very short" - does this mean that it will be pulses of 12V even with just power or just a general statement overall? Not really sure how to read into that one.

Tested the pressure sensor and I have a feeling its bad. We were only getting ~2.8V with key on, port plugged. Is there a way to confirm either that this sensor is the issue or if it isn't, simulate load and verify the secondaries are working/firing? My thoughts are this should work:

1. rev engine to 4000 rpm
2. Pull vacuum on pressure sensor to simulate load - with a syringe we can pull up to 50cmHg on a vacuum line.
3. Engine sound should in theory change once secondaries kick on.

Should an S5 NA engine have a "variable resistor"? I do not recall seeing one in my many days spent within the wiring diagrams. I currently do not have one so if this is needed, that may be a concern.