Engine Ground ? Stumble at WOT

[DavidSalama]

SOLVED!! See later posts... AND VIDEO!!

Hello, My Noble M12 recently had a clutch job (engine out) and ever since, I have a major stumble at WOT. Part throttle is perfect. Dyno shows rear bank going lean 15:1 with stumble. Front bank stays rich at 11:1. I have checked nearly everything. Injectors swapped, plugs changed and gapped to 0.028, new coils, fuel pressure checked 42psi up to 50 psi with boost, new fuel filter, new o2 sensor in rear, and smoke tested for air leaks. My Map sensor checks out, and my throttle position sensor does show a brief open circuit at 3.2mv. I got a new sensor, and the exact same open circuit occurs.???

I am now focused on the ecu, and trying to figure out if everything is grounded properly with the engine out and back in. Does the engine have a ground cable somewhere?

I have performed a compression test and leak down as well and all is perfect.

Part throttle under 5 psi boost is smooth and strong up to redline.

Thanks for your help/comments/recommendations.

Dave

Edited by DavidSalama on Wednesday 26th October 13:03

Edited by DavidSalama on Monday 7th November 14:02

[sjc]

I'm not sure I'm understanding all of that in translation, but if it is only happening at full throttle, have you upgraded the fuel pump wiring? The voltage running through it can drop, causing the car to run lean, and be hesitant for a split second under full throttle.It doesn't seem to happen under part throttle.

[andygtt]

supply hose to the fuel pump has been known to collapse under full load causing exact symptoms described above... check the pump wireing and the fuel hose from tank to pump very carefully.

[TuxMan]

Hi David , interesting problem as I would have thought if fuel pump or pipework was the issue it would effect more than just one bank , still worth checking though .

There should be a earth strap from engine to the rear engine mount behind the exhaust , it does get in a bit of a state from the heat so it might be worth replacing it .

While clutch job was being done did the battery go flat ?
I would speak to Yellowshed ref ECU must be ECU related as I see no other reason why one bank will go lean .
Tux

[funkybmx1]

Also check your relays on bulk head. Mine ran on one bank once all cause off a relay. Just something else for ya to think about

[DavidSalama]

Thanks for all the input. Which relay is for the rear bank? I have changed my collapsed u hose between fuel pump and filter already to a stainless steel line. I have not upgraded the wiring to fuel pump yet, but it was fine before engine out... I will look for the engine strap, I did not see it looking around. Is it under the rear turbo area? When all else is checked, I will send my ECU back to Hoover to check. The battery did go flat while engine out. Any suggestions based on this knowledge?

Also, has anyone done multimeter readings off the injector wires? What should they read?

Thanks.

Dave

[hollowpockets]

The engine earth is under the lower rear engine mount, you should be able to see it by opening the clam and looking in under it from the passenger side.

You should get 12v at the coil plugs, upgrade the wiring to the fuel pump is a must do even if it's not the cause.

[DavidSalama]

I will look for the ground strap..

Is there an upgrade wire harness for the fuel pump, or just replace the wires with better gauge? Do the wires go to the relays on the bulkhead?

Also, does anyone know the multimeter reading on the injector harness wires?

Thanks!

[Gadgeroonie]

has the car been remapped ? does it run std boost ?

i suggest you take out all the plugs and have a good look

might be worth reducing the gap on them slightly

[DavidSalama]

I have changed the plugs to colder plugs. i am running 1 bar boost, but with stumbling I only get 7-8psi. I will try to reduce gap even further.

Thanks.

Dave

[DavidSalama]

Cleaned up the engine ground strap today but no change. Decided to do a little experiment with my air fuels to see if my lean readings are actually from raw fuel in the exhaust. I disconnected the vacuum line from the fuel pressure regulator. Ran the car with the wide band o2 sensor on the rear bank. The car ran a bit better, and I was able to ease into full throttle and get great power with an air fuel closer to 12 rather than the upper 10's low 11's.

Now I am thinking that I am washing out the spark causing the sputtering. I am going to send my ecu back to Hoover to get re-flashed with trimming some fuel out. We shall see.... I hope this cures my Noble!!

[joust]

You have checked & narrowed your spark plug gaps? Sounds just like the issue I sometimes have with a new set of plugs even though they are set at the recommended gap.

J

[DavidSalama]

I will do that when I get my ecu back from Hoover. I know it's running real rich at least in front bank, probably rear also, so trimming fuel will help with the spark as well.

I will run with new flash, then pull plugs and narrow gap further.

What do you think about a hotter plug? I went colder from a NGT Iridium plug #8 from a #7 standard plug.

Thanks.

Dave

[DavidSalama]

Correction,

My new plug are NGK TR7IX not 8... Old plugs were TR6 copper.

Dave

[hollowpockets]

I see you mentioned iridium plugs, For what it's worth yellowshed took the iridium spark plugs out that had been fitted to my car the first time I saw him, apparently the spark from them isn't well suited with turbo'd cars.

[DavidSalama]

Alright, so I get a call from Hoover today. He has my DME to check out, possibly trim fuel levels, and reset adaptive maps. He tells me the adaptive maps are adding a ton of fuel randomly and he cannot reset it. He tried in several different ways to reset, but no luck. He thinks this is responsible for my stumble issue. Outside of buying another DME, he turned off the O2 sensors to hopefully take the adaptive mapping out of the picture. I will update when I try it out.... I hope we have discovered the problem.

[DavidSalama]

SOLVED!!!!


After over a hundred hours working on this stumble, I found the problem. I had a short in the number one coil plug signal wire. My mechanic had rewired this months ago, but I finally tore down the insulation and looked at it. The original wire was soldered to the new wire on both ends, plug and ecu end. Once I clipped the original wire on both ends, the stumble went away. Before getting here, I tried the following. Smoke test, vacuum line testing, new plugs, new coils, MAP sensor tested,fuel pressure, new crank sensor, injector swap and spray test, compression testing, dyno testing with wide band, throttle position test, waste gate testing, sent ECU to Hoover for check up, downloaded Easimaps, and bought 9 pin connector from Troy. Troy spent many hours helping me from Florida and was awesome. His knowledge of the mapping/ reading logs is unbelievable. Hoover was also great and his flash on my Hooverized Noble is spot on.

After all this, you guys on the forum were correct. Installation issue, wiring shorted after engine R&R for clutch job.

Just need to clean up my wiring and I am going to have some happy motoring ahead (after a year of Noblelessness) Yeah!!!!!!!

[DavidSalama]

Video of a couple of runs at the Porsche Club Autocross in Charlotte NC. Noble is running great! Fun times ahead...

http://youtu.be/6tOFNA_s2eQ