Monday, August 20, 2007 - 09:29 pm, by: Jeremy Abram(Jem)
Hi guys, first post here!
Ok my V8 runs fine when stone cold and starts first time everytime. But once it's warmed up, the engine hesitates and feels like it's missing under load. But it idles smoothly and on full throttle over 3500rpm it's fine. It's under normal acceleration that it's most noticable.
If I disconnet the air flow meter the engine runs much better with great acceleration and really highlights how bad it is with the AFM connected. The fact it runs much better with the AFM disconnected should rule out the ignition system and suggests the mixture, correct?
I've tried another AFM and it was just the same, although this AFM was unknown if it was working properly. Whats the chances of two AFM's being faulty?
I've tested the voltage from the O2 sensors with a multimeter and got the following:
Starting with the engine stone cold, as the engine warms up the voltage goes from 0.00v and to about 0.50v when the engine was roughly half warm. It then sits at 0.05v and jumps to about 0.50v every seven seconds. This was at idle and I'm not sure it should do that at idle. But if the engine is held at a fast idle, the voltage jumps from around 0.20v upto around 0.90v roughly every second, which my understanding says is what they are ment to do. I'm gussing that at idle the ECU will run a very lean mixture to save fuel, hence the 0.05v with a jump to around 0.50v every seven seconds.
Also if I disconnect the air flow meter (which makes the engine run much better, with no hesitation) the voltage rises upto 0.9v and pretty much sits there, suggesting a rich mixture as would be expected.
So it appears my O2 sensors are working as they should.
Any other suggestions please, as I'm getting to the end of my tether with this now!
Monday, August 20, 2007 - 10:53 pm, by: Damian Ware(Frozenpod)
Under full load above 4000rpm the ECU determines the amount of fuel to add by using AFM. Unless the mixtures are completely wrong under full load I would suggest the AFM is operating ok.
Below 4000rpm the engine will be running in closed loop off the oxygen sensors but with reference to the AFM and air temp sensor and water temp sensor.
If any of the sensors are incorrect it can cause the ECU make corrections which it thinks is correct that could cause the AFR to be rich or lean. From your description it sounds like it is running lean in closed loop.
As above I doubt the AFM is the problem as if the AFM output is incorrect the engine will run rich or lean but it will adjust itself very quickly and run of the oxygen sensors somewhat ignoring the AFM (depending on how incorrect the output is).
I would be checking the temp sensors and if they are operating correctly. I would also disconnect the oxygen sensors and if possible look into the ECU. A faulty ECU can cause the AFR to be incorrect.
Replacing the oxygen sensors is fairly easy and they are cheap so if you cant find the fault I suggest replacing them.
Friday, August 24, 2007 - 11:23 pm, by: Jeremy Abram(Jem)
Well the new O2 sensors are in (after much faffing about. One come out very easily, the other I had to re-tap the hole in the exhaust, but they are now in.
But the engine is running exactly the same, missing under load up to 4000rpm but fine above 4000rpm.
Saturday, August 25, 2007 - 02:06 am, by: Jeremy Abram(Jem)
This is me (at the back to the left hand side):
Well that will teach me to over look the fecking obvious!
This afternoon I pulled all the plugs out. In my defence I had been told by the seller that the plugs had been checked and were all ok. Infact the plug gaps ranged between 1.1mm to 1.4mm. They should be 0.8mm. Also the plugs look like they are passed their best. The colour of them is good though, a nice light brown.
Two of them did have evidence of gasses passing between the metal and the ceramic which could be seen as a 'burnt' area of the ceramic where it meets the metal casing. A couple of the HT leads were showing signs of arcing too.
I closed to gaps upto 0.8mm, and now it's running perfectly.
so I'm going to order a set of new plugs and new leads:thumbs:
Friday, September 14, 2007 - 03:44 pm, by: Gregg Holden(Xzotic)
Shame I didn't see this post earlier. Had exactly the same problem - Joe "Hollywood" immediately suspected arcing of the HT leads. This was confirmed, replaced and all sorted within a couple of hours.
Adam Peterson Goo Roo Western Australia Bugatti Veyron
Monday, September 17, 2007 - 07:44 pm, by: Gregg Holden(Xzotic)
No adam no pulsing on coast, however once you ease the throttle when cruising you get the stuttering problem as the leads start to arc. When you rev up past 3000 odd then it goes away but will repeat again anywhere in the lower power band.
If yours is pulsing or surging then may be a different problem (or a combination of issues - HT leads being one of them). You said your HT leads showed signs of arcing - have you replaced them yet?...
Adam Peterson Goo Roo Western Australia Bugatti Veyron
Tuesday, September 18, 2007 - 11:09 am, by: Adam Peterson(President)
I havent replaced anything yet. I have only had the car for about 5 weeks and its riddled with problems that is damaging my wallet! I have already had to spend over $1600 just on cosmetic stuff alone and i still have a lot to go. It may be spark plugs ? Plus i dont have a service history either so i dont know whats happening.