Engine Stuttering Under Load

[fieldy107]

I think my problem has gone beyond "stupid questions" so I'm making this thread to try to figure out this problem.

I'll take it from the beginning...
Last Saturday I had the car flash tuned by Merlin. Car had no issues I'm aware of prior to this day, apart from poor fuel consumption. Car ran fine that day during the tuning until I headed home later that night, I go to accelerate hard from 80-120 in 3rd gear and the car starts jolting, as though its hitting fuel cut, although not as violent as people describe fuel cut. At first I thought it was boost cut but after putting boost down to 12psi it still happens, when previously I was running 14+ psi without issue.
Once this problem starts occurring it will do it over and over no matter what the gear, just gotta give it some boost and revs. Testing it on short trips it does not seem to happen.

earlier In the week I posted my error codes
Engine
1A No.1 cylinder accidental fire detection
ABS
21 FR wheel speed sensor system
AYC
41 TPS system (short or break)

After resetting the codes and driving until the problem showed itself I have checked the codes again
ABS
- 21 FR wheel speed sensor system
Which I think is unrelated to my current problem? surely...

I think my accident cylinder fire might of happened because I kept my foot on the gas to long while my engine was jolting, forcing a miss fire. So the jolting isn't actually a miss fire itself.
I was really surprised that I did not get the TPS error again, as over the last week my car has idle has been hunting and usually settling at about 2k rpm, and it also feels like its still on throttle slightly when my foot is off the pedal. I was sure this was my throttle position sensor, but no error!?

So to sum it up, car is jolting under load at highish revs causing a miss fire in the worst cases, idle is all over the shop and I'm not getting any error code to tell me whats wrong. help

Let me know if I have left out any important information

I'm not doubting Merlin's skill as a tuner as I have witnessed for myself how well he knows our cars, but I have to ask the question, Is it possible a mistake has been made that causes this problem? A miss click or wrong key perhaps? If you read this Dave, let me know as I would like to rule this out as a possibility.

A little info about my car
99 facelift auto legnum type s
Stock apart from boost T running 12-13 psi

 

[king_panther]

Have you tried sending Merlin a PM Chris? He doesn't really spend time browsing through the forum...

Sounds like you've definately got some gremlins in there though. Shame...

 

[fieldy107]

Not yet, I'll do that now.

 

[naughtika]

my suggestion is to try and replace your spark plugs..

I know you've only recently changed it.. do you know what the gap was set to? I know everyone here sets it to .8mm, also i usually double & triple check the gap just to make sure.. cause we all know its PITA to pull the rear plugs out again..

do you know what revs you were at when the stutter/jolt is happening?

 

[fieldy107]

Plugs were bought from Kudos Motorsport (a supplier on here), I assume they were gapped correctly as I made it clear they were for a legnum and are less then 10k kms old. Also this time around there was no engine miss fire error, so makes me think its not plugs?
The jolting starts about 4500rpm I think and doesn't stop until I take my foot off the gas, the car doesn't really accelerate when the jolting is happening.
What are the symptoms of a bad fuel pump / injectors? it almost feels as though the engine isn't getting enough fuel, but this would trigger a different error code I guess?

 

[Buckas]

i also bought my plugs from a trader and they weren't pre-gapped to the claim...

 

[naughtika]

sounds like plugs to me.. usually happens when spark plug gaps are too wide, compliance recommends gapping it at 1.1mm and standard BKR6EIX and BKR7EIX are set to 1.1mm as well..

you'd be amazed what that .3mm spark plug gap difference would make..

also if its the pump, should start as soon as you hit boost, not while boosting..

I've got another set of injectors here if you'd like to try it out.. but I doubt its that, you'd smell petrol while you crank to start your car cause it'll flood the combustion chamber with fuel..

and yes, error codes would show up too..

 

[godzilla]

Chris, what happens if you wind the boost controller all the way back to stock? IMO, if doing that, and the car still splutters, i wont be plugs. There is no reason why the plugs will fail between >boost up>tune>then boost down. The plugs would continue to fail under boost levels higher than stock if A) they are simply the wrong plugs B) the gap is wrong. The plugs fail because the boost "blows out the spark" so-to-speak.

I am thinking coil packs. I am not sure if a coil pack would read an error code if they are just dirty etc, would they?

I don't think your fuel pump will be shot as your car had well and truly enough fuel during tuning. Dave should be able to confirm that. I think surging may occur if the fuel pressure reg is playing up though.

I just thing you should do a couple of things. First, wind the boost all they way back to stock and go give it a strap. Then, remove the front three plugs and review the gap. If you bought the plugs from the same place they "should" all be the same gap.

Edit - As Christian has just posted. If the gap is to wide, you will get blowout under higher boost levels. The wider the gap can be good but under moderate boost.

 

[naughtika]

i remember when I changed my plugs, it was set to 1.1mm and during boost (14psi).. it would stutter as well.. I removed it and gapped it to .8mm and was fine after that..

but after 50k kms, rev was stuttering at around 4500rpm upwards.. indication that I need to do my plugs..

 

[Macca]

Just a thought, it could be a partial blockage in the fuel filter. During normal driving its fine but under demanding loads it cant get enough fuel. When was the last time you changed the fuel filter?

Just a suggestion.

 

[godzilla]

Chris changed his filter about 4 weeks or so ago.

 

[fieldy107]

Yep as Trev mention'd fuel filter done not long ago.
I will put boost back to stock and see what happens. Maybe the gapping is wrong and the spark plugs didn't mind the higher boost before the tune as the engine was being so over fueled it just handled it anyway? Now that I'm running correct fuel ratio's it causing the plugs to fault? thinking out loud...
Think I will get the coil packs cleaned and the gapping checked as well.

 

[fieldy107]

Dave mention'd my high/hunting idle is probably due to a bad idle speed control valve. This sound about right? its only really been happening since I did my ECU reset, so I think its a whole different unrelated issue to my engine jolting.

 

[naughtika]

Chris, let's try and eliminate the stuttering first then we'll focus on that high idle, you never know it might just go away after getting that stutter solve..

 

[merlin]

Christian has a made a point of the plug gap, pulling the plug gap down to 0.8mm can often give a fix to hi-boost spark blow-out.
Or, if there is some weakness ing the ignition setup. They are old coils..........

Measure the plug lead resistance.

Check the plug gaps.

Check the vacume hose going to the fuel pressure regulator, this is prone to popping-off on boost on Evos.
If it has popped-off, zip-tie it back on. This applies to all VR4s running above stock boost guys.

I have checked you ROM Chris, nothing amiss there that I can see. A suggestion re the tune would be to swap ECU with Trevor and do a hot lap and see if the problem is still there.

At the moment I'm thinking coils/leads or that damm regulator hose. Your AFRs were spot-on during tuning, so I dont think its pump or filter related.

 

[Macca]

Well if the filter is new and the logs show no problems with the fuel system the like everyone else has said could be ignition related.

 

[justdo]

Just going to through something else out there,did you loosen any of the intake/intercooling clamps off?had the same problem on my gtr familla a couple of years ago,absolutly had me buggered & I just stumbled across it,tensioned them up problem solved.Just a thought.

 

[fieldy107]

After my spark plugs were done (months ago) I had a slight rattle, so I took it back and made them tighten up all intake piping, so I doubt that is the problem in my case. thanks anyway

 

[godzilla]

How is your fuel consumption so far?

 

[naughtika]

Chris, before you pull out all the spark plugs, I'd say pull out any of the 3 from the fronts.. and if the gap is correct, check the another one..

oh and service your coil packs too while you're at it..