Odd engine behaviour, mixture issues

gillis

Junior Member
Messages
77
Since the beginning of my ownership of a 4200 CC I notice some things I want to address. It is not a deal breaker, but it annoys me. I drove the car about 20k miles. Currently 70k miles on it.
When i start my car cold it idles fine. You can hear and feel some tiny oddities though, which I think are misfires. But it's hardly noticeable. Some people say it's normal. No cel comes up.
When I fully press the acceleration pedal while standing still in neutral it looks like the mixture is not correct. Eventually it will rev up, but it starts with second of low power output. As if too much air or fuel is injected resulting in rumbling, shaking noise. Just pressing it a bit shows no issues.
When driving, this issue also occurs in the low <3k rev range.
Sprinting from a traffic light goes perfectly fine. Power is all there in the high revs.
Then sometimes when I do a warm start the rpm goes up (normal) but then falls back to below 900. ECU corrects this and directly afterwards it falls down again 1 or 2 times before it idles normal. This happens not always. Most cases it's fine.
What would you guys start investigating first? Coils, injectors, vacuum leak or something else maybe?
I couldn't find any error codes from my reader.
 

BennyD

Sea Urchin Pate
Messages
14,994
Use Iridium plugs. Some say don’t but mine have worked perfectly. If not the plugs, it’s probably a bit of Italian character coming out. Thrash the f*ck out of it and either it will get worse, or get better; then you will know which way to go.
 

lozcb

Member
Messages
12,301
Agree with Ian, although these are gentlemen's sports cars ,they need a thug at the wheel every now and again , that V8 needs airing clearing rather than too much 40mph posing ;)
 
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hladun

Member
Messages
149
I'd start with a bottle of fuel injector cleaner and I'd clean the MAF sensor and throttle body.
 

FIFTY

Member
Messages
3,100
What about a vacuum leak? Check the intake thoroughly and then there is a number of vacuum hoses
 
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lambertius

Banned
Messages
341
Plugs were done about 5k miles ago.

If you don't have a basic OBD reader, you can get a cheap one at most auto stores or on eBay for about $15AUD. I'd start there. A lot of the sensors that could cause your issue will throw codes.
 

Zep

Moderator
Messages
9,110
The vacuum leak is a good shout. At your mileage you could also be on the cusp of an issue with the lambda sensors. They get lazy (the cycle time increases) and you get an increase in the long term fuel trims.

This has been known to cause rough running before it reaches the threshold for a fault code or CEL.
 

wicked1

SPONSOR: Giallo Cars Ltd
Messages
34
We recently has similar 4200Gt with similar characteristics and it was down to the throttle body filthy. When stationary, you could not hold the throttle between 2000 to 2500rpm, the engine speed would either drop down below 2000rpm or rev up beyond 2500rpm. It became apparent when we couldnt get a fast idle test on the MOT. We cleaned it thoroughly and did a throttle reset, which not only solved the slight odd idle but transformed the throttle response when driving.
 

Kyle4200

Junior Member
Messages
251
Since the beginning of my ownership of a 4200 CC I notice some things I want to address. It is not a deal breaker, but it annoys me. I drove the car about 20k miles. Currently 70k miles on it.
When i start my car cold it idles fine. You can hear and feel some tiny oddities though, which I think are misfires. But it's hardly noticeable. Some people say it's normal. No cel comes up.
When I fully press the acceleration pedal while standing still in neutral it looks like the mixture is not correct. Eventually it will rev up, but it starts with second of low power output. As if too much air or fuel is injected resulting in rumbling, shaking noise. Just pressing it a bit shows no issues.
When driving, this issue also occurs in the low <3k rev range.
Sprinting from a traffic light goes perfectly fine. Power is all there in the high revs.
Then sometimes when I do a warm start the rpm goes up (normal) but then falls back to below 900. ECU corrects this and directly afterwards it falls down again 1 or 2 times before it idles normal. This happens not always. Most cases it's fine.
What would you guys start investigating first? Coils, injectors, vacuum leak or something else maybe?
I couldn't find any error codes from my reader.

I had similar issues with mine, I replaced all the spark plugs to no avail, turns it out was a dodgy coil pack, replaced it and it was fine, it did show misfires on different coils occasionally on a scan tool but then all went away once I changed the dodgy one out.
MAFs can play up also, when’s the last time you changed your air filter? Not that this would have a ‘massive’ impact on your symptoms.