Help! Bouncing RPM when reving past 1750rpm when car is stationary on stock 2000 RS

[ascari]

I've been wrestling with this and have run out of ideas. This is not the idle hunting issue. It idles at 1200, lower when it warms up, but the moment I press the accelerator, the engine will lose power, drop revs, then bounce back up to the target RPM, and then repeat. If I hold the RPM above 4000, there is no problem.

There is no problem when driving, so only happens when stationary, and there are no ECU error codes.

When I do open the taps on the road, it's like a missile!

This is what I've done to try and fix it:

1. Replaced oxygen sensor (weirdly if i disconnect it, there is no ECU error code!)
2. Refurbed ACV
3. Replaced all vacuum pipes
4. Tested all solenoids
5. Tested all 1-way valves
6. Checked boost sensor
7. Checked all spark plugs - brown/black ends
8. Reseated all grounds
9. Disconnected/Reconnected battery to reset ECU error codes (there weren't any anyway)

This issue didn't happen previous to a large amount of work I've done over the past few months (twins replacement, LIM gasket replacement, coolant hose replacement, solenoid test and replace, full vacuum hose replacement, OMP redirect to oil filler neck - unusual I know, but I premix now and wanted to leave the OMP in place).

I wonder if this is limp mode, or if I have put a vac pipe on wrong, but I was painfully careful to follow the FSM vac diagram.

Maybe my OMP mod is confusing the ECU, but I can't see how as it is essentially operating as if it were connected to the housing oil injectors.

Any suggestions greatly appreciated.

 

[MKZ7212]

Stock engine, stock ECU?
Why did you move the omp lines?
I premix and run the OMP
Did you block the OMP lines into the housings?

 

[ascari]

Stock engine, stock ECU?
Why did you move the omp lines?
I premix and run the OMP
Did you block the OMP lines into the housings?

Totally stock 2000 RS (I've updated the title)
The OMP lines cracked, I bought replacements and ended up concluding I wanted none of the potential failures of OMP oil injection, so 100% premix. I realise there is a lot of debate on this topic...
The oil injectors have been removed and are blanked off.

 

[MKZ7212]

Check all the adjustments around the TB, is there slack on the accelerator cable, think about the stuff you moved around that area (presuming you didn’t alter the idle mix screw)

 

[ascari]

Check all the adjustments around the TB, is there slack on the accelerator cable, think about the stuff you moved around that area (presuming you didn’t alter the idle mix screw)

There's 2mm slack on the accelerator cable, and I left all adjustments on anything I took off alone, especially the idle screws, as i aim to attack that after this issue is fixed.

The things I've taken off/on that could be put back are vac pipes, but i was very careful, used photos, took my time. Can it be a vac related if the car drives fine?

 

[MKZ7212]

could be vac related, did all the one way valve go back on the right way?

 

[ascari]

could be vac related, did all the one way valve go back on the right way?
View attachment 15792

I'm sure they did... gonna have to double check the lot I guess.

 

[ascari]

This may be relavent, before I worked on the car, if I accelerated up to 2750RPM, it would suddenly drop power/revs to around 2300RPM, but you could accelerate through it. I believe 2750 is when the secondary fuel injectors come on.

 

[MKZ7212]

pull the elbow off and make sure the throttle plates are not sticking.
secondary injectors come on around 3k depending or ecu readings (temps, load etc)

 

[ascari]

pull the elbow off and make sure the throttle plates are not sticking.
secondary injectors come on around 3k depending or ecu readings (temps, load etc)

Throttle plates aren't sticking, the 2 butterfly valves (if that's what they're called), which are controlled by the little actuator on the side of the UIM remain open during the issue.

 

[MKZ7212]

if it was ok before it must be something you have not put back right or moved, either that or the ECU
been there myself and had to triple check stuff before finding the fault

 

[ascari]

if it was ok before it must be something you have not put back right or moved, either that or the ECU
been there myself and had to triple check stuff before finding the fault

i agree. i hope it's not the ECU. if i disconnect the oxygen sensor, there's no error code, nor for the purge control solenoid, but it does show error codes for the turbo & wastegate solenoids... hope that doesn't equal fried ECU.

 

[shaunwil]

Have you done anything to the vac pipe to the brake solenoid, sounds like a leak.

Have you rebuilt the throttle body, could be a leak there too.

 

[ascari]

Have you done anything to the vac pipe to the brake solenoid, sounds like a leak.

Have you rebuilt the throttle body, could be a leak there too.

i've checked the 1 way valve in the vac pipe connecting the UIM to the brake master cylinder... i will check the pipe itself holds pressure now tho

 

[ascari]

Have you done anything to the vac pipe to the brake solenoid, sounds like a leak.

Have you rebuilt the throttle body, could be a leak there too.

brake master cylinder vac pipe to UIM is ok

 

[ascari]

Checked all vac pipes (again), all good.

 

[Adam]

Unplug the iac valve to rule that out

 

[Mid Night FD]

Had similar problem on mine, it was really intermittent. I found if i moved the car at all before it got up to temp it would happen more often, also if id heel/toe on downshifts it would start bouncing. I never found the issue, replaced all vac lines. If i played with the throttle plates and actuator it would sometimes calm down and idle normal again. In the end i went Apexi Power FC Ecu as i planned to modify and it has never done it since.

 

[JoeLanger]

maybe try a know good ECU, someone on here would have one knocking about

 

[ascari]

Unplug the iac valve to rule that out

i'll do this on the morning