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Discussion Starter #1
I was driving up towards the alps ( San Bernadino pass). On my way up there I got misfire on the left engine ( codes read misfire 6+7+9+11 I think). Quite bad. Thought first is was a loose cable or something ( but because more cylinders were involved it was not likely to be a fault on a singel coil or sparkplug).
I didn`t know what to do so I drove on and through a tunnel and downwards. With a lot of throttle ( 40 %+) the misfire went away. Low RPM and high gear gave more misfire.
When I got Down the engine suddenly started to run nicely again. I do perhaps feel like there is a small problem still. It might even have been there for a while but I haven`t noticed. Low RPM and 5 gear and push the throttle = the engine have a mini-misfire before everything sorts out at higher rpm. Almost so you don`t notice.
Any thoughts on what this can be? My mechanic thought the engine was under- or overcompensating the amount of fuel based on the oxygenlevels.
 

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As far as I know, there's no way sensor to compensate the outside pressure, only manifold pressure sensors (located connected cross-wise (LH engine - RH manifold - LH MAP sensor)).
I think you're throttle bodies might be out of perfect alignment, so you get higher MAP reading on one bank, which leads to leaner mixture at high altitude and you get a misfire.

However, I'm speaking about pre-96 injection, 6.0 VT may be a little different.
 

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Yes,
You have issue with manifold air balance, that whent over to "missfire" when altitude and lack of oxygen caused it.

btw, there are inlet temperature sensor.
 

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btw, there are inlet temperature sensor.
Ah, right inside the manifold covers? I forgot, as they are on the engine itself...

But anyway, there's no air density sensor, which is good to have sometimes. Subarus have those, just a MAP sensor opened up.

Also my friends were building a hillclimb race car they used the same trick to avoid high altitude mixture problems, they used again a MAP sensor that was vented to the atmosphere to correct air density. Later the car came 1st, btw. :wave:

It was carburated before and tuning those carbs for high altitude was quite problematic, so they finally gave up the Webers for an EFI.
 

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My 99 will pretty much always get codes thrown with substantial altitude changes eg Lake Tahoe. Never seen any misfires but fuel trim related codes for sure along with a noticeable loss of power. All very temporary.

-mick
 

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Discussion Starter #7 (Edited)
I now have even more problems: it now has constant misfire at low rpm and engine code P0174 ( left bank running lean). I have checked the fuel trim With OBD2-reader and it tries to correct the fuel by 20-30 %. At higher rpm the fuel trim stays at 25 %.
I believe ( not sure though) the new problems are the same as the old but worse. I had the feeling back when I posted this thread, that i didn`t have full power.

I haven`t seen any loose hoses or anything like that. I think all the sparkplugs sit correctly ( they were all checked 8 weeks ago by my mechanic).

So this is consistent With bad air Balance? How do I adjust it? Is there a screw that I can adjust and in real-time check the OBD2 to see if the fuel trim goes Down to 0-10 % ( as the fuel trim is on the right side)??
 

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Discussion Starter #8 (Edited)
lean fuel trim p0174 and p1602 misfire

OK, more info to the guys who know what it means….:

The codes are p0174 ( Fuel System (KTYTRIM over +0.75) too weak (cyl 7/12)) and p1602 ( CAN LINE Communication Fault).


The live parameters at idle ( if I have them all):
DTC_CNT= 2
Fuelsys1: OL-Drive
Fuelsys2: OL-Drive
Load_PCT (%): 6,7
ETC ( Fahrenheit): 118
SHRTFT1 ( %): -8,1
LONGFT1 (%): 1,6
SHRTFT2 (%): -21,9
LONGFT2 (%): -25,0
MAP ( inHg): 13,6
RPM : 1105
VSS ( mph): 0
SPARKADV (degrees?): 10
IAT ( Fahrenheit): 59
TP ( %): 6,3
O2S - B1S12 --B2S12--O2B1S1: ( hard to read….0,something)
SHRTFTB1S1 ( %): 1,6
O2B1S2 (V): 0.760
O2B2S1 (V): 0,895 ( i think it says)
SHRTFTB2S1 ( %): -12,5
O2B2S2 (V): 0,510

By the way: I tried to shorten the rod that went back to the air inlet manifold by rotating the thing at the end ( the Connection between the rod and the mechanisme back there). That didn`t change anything. Was hoping to influence the air into the left side of the engine by doing that. Have restored it back to the way it was now so no harm done!!
 

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Where did you get the live parameters btw?

Anyway, first impression:
- pretty high throttle openning (wonder if throttle position sensor is the same)
- O2 signals are wacky, seem to bee too rich?

- injector problems possible (due to different O2 signals on one bank)
- unequal length throttle rods possible
- TPS adjustment might be needed too (need to check 6.0 manual)
 

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Oh, it's cool that it shows so much info.
Earlier ones are like James Bond interrogating an epic criminal - they only tell you some flamboyant jokes and you have to guess the rest. Unless you have an LM console.
 

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The codes are p0174 ( Fuel System (KTYTRIM over +0.75) too weak (cyl 7/12)) and p1602 ( CAN LINE Communication Fault).
Really need the codes along with the conditions. If there was a CEL *during* a major elevation change and that CEL was P0174, then this maybe just fine. The code would have been stored and the CEL would auto clear after some time at elevation. However, that's not what started this thread which was misfire codes during elevation changes. So where are we - when did P0174 occur and what happened to the misfires?

-mick
 

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By the way: I tried to shorten the rod that went back to the air inlet manifold by rotating the thing at the end ( the Connection between the rod and the mechanisme back there). That didn`t change anything. Was hoping to influence the air into the left side of the engine by doing that. Have restored it back to the way it was now so no harm done!!
This you can not do,
If those rod locks (nut) is losen, compleat adjustment is out of the window, you can not touch those parts if you do not have syncrometer (manometer) next to you and you have to start adjustment again.
This balance adjustment has to be done before actual issue can be found, othervise it will mess up any information you need to find issue.

Although I suspect air balance is the issue here, see that all vacuum hoses are not split and attached to ports and so forward, you do not have a leak anywhere.
 

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Discussion Starter #14
I just thought that the misfiring in elevation is connected to the problems I have now. But now its much worse and the car doesn`t go well at all. Can`t be driven now, practically.

The error codes and problems I have with the engine now are under normal conditions ( sea level and nice summer temperatures) and they are constant .
 

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Ok, welcome to DIY Lambo community then, lol.
You can retune the throttles yourself, if you disconnect the linkages and synchronize them with bypass screws, then you put on the linkages and synchronize them, so that they are the same as they've been.
Also you need to watch throttle position sensor (TPS), maybe it needs adjustment.

To watch the synchronization you can prepare connections to connect between harness and MAP sensors, so that you can permanently hook up a multimeter (voltage tester) on both sides of the engine. Then its easy to sync them, watch the idle and etc.
You may try to hook up to check the TPS Voltage, although if you have OBD, you may not need to check voltage directly.
 

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Discussion Starter #16
lean fuel trim p0174 and p1602 misfire

From the mechanic after a long time troubleshooting:
"Short summary: Car arrived with engine warning lights and engine not running well. A few minutes after every upstart the engine went into emergency mode and started to give left engine bank (7-12) too much fuel. However fault code indicated "fuel system too lean, bank 2". Fault search found out exhaust pressure sensor 7-12 had a bad connection. After that was fixed the car no longer went into emergency mode and no longer gave left engine bank too much fuel. But fault code remained. After extensive fault search mechanics found a bad connection in the main switch. This was particularly difficult to find since no voltage drops could be either manually measured nor seen with the diagnostic tool - not even when cooling fans started. After main switch was replaced fault code disappeared. Another thing that was misleading the fault search was that both banks had leaking exhaust manifolds. On (at least) European Diablo VT 6.0 cars leaking manifolds often give these type of faults."
 

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Discussion Starter #17
Problem solved!

After 1,5 years ( ok, we just parked and forgot about the car for a while…) the problem is solved.
It was the wires to the exhaust pressure sensor on the left side that was damaged from heat. They had melted into each other. It was not easy to see and the wires where tested ok earlier on. But it must have been the issue. Now it runs fine after a 30 cm lenght of the wires where replaced.
 

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Glad it is fixed mate. Is the exhaust pressure sensor the oxygen sensor, or is it something different?
 
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