1996 GTS w/P0175 getting about 150 miles per tank of gas (was 300)

[Tom and Vipers]

I'm about to start diagnosing this today and my #1 suspect is a leaking injector O-ring.

The first thing I'm going to do is inspect the spark plugs and see if there is evidence of a rich injector. I shut the engine down while presenting the 3000 rpm idle.

Once I do that, I'm going to listen to the injectors while engine running to see if they are clicking as expected.
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Other thinking:

• Since the banks are managed independently by the ECU, a lean condition in Bank 1 could not be a cause.
• Intake vacuum leak not expected since this would cause a LEAN code.
• Sometimes when I start the car, the idle instantly spikes to maybe 2000 rpm and then instantly recovers - as if someone put a shot of starting ether - fuel puddle in intake?

Other information: High Idle or Hang: I don't know if these are related but I suspect they could be. So here is the latest thinking on the idle problem:

I've been dealing with a severe idle "hang" as high as 3500 rpm which has happened at altitudes around 8000 feet. But when extreme like this, the "hang" never drops, it just stays there. It will wander or hunt around this level sometimes with step changes in rpm which could imply a bad electrical connection. It has VEC1 and I don't think the connections were soldered - just pierced. But I don't think its the VEC1 because that would effect BOTH banks.

Yesterday, it went into "3000 rpm idle mode" from 6000' down to 4800' which is contradictory for an altitude correlation.

HOWEVER, there does appear to be a correlation with temperature: temps in the 50's may be the trigger for this mode.
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EDIT UPDATE: BLOW OUT THE PLUG HOLES BEFORE REMOVING! They are FULL OF SAND here in the desert. DUH!

EDIT UPDATE 2: Checked plugs, looked good. Both banks same, no single plug showing rich.

Hot again, around 80*F, VEC @ 0/0 Low/High and the high idle crept in as soon as engine warmed up. It would creep to 3000 rpm at 4800 ft. I left it go in neutral and it would hold high for 5-10 seconds, then drop like it was going to idle with the exhaust "gurgling" BUT around 1500-2000 , the drop would abruptly stop and again creep back to 3000. This was an oscillation! I would repeat this indefinitely.

So I started adjust the VEC1 Low. I went lean and it got worse, I when something like 2 clicks rich rich and it dropped to idle and then behaved itself.

I wonder if this oscillation is the "confusion" Cragin was refering to in my high altitude thread.

RULED OUT:

• Injector o-ring leak (no single plug presented rich - of course, if one injector started dumping fuel, how rich would the other cylinders present in that bank? I don't know.)
• Intake vacuum leak (how do you make a bank go rich with a vacuum leak on that bank?)

NEXT STEP:

• DRB the codes and look at the plot O2 and fuel trims.

THINKING:

• The heart of Speed Density may be the MAP sensor. Clearly the VEC1 has a dramatic effect on idle. Of course on my car, this behavior may be a symptom of something else - I don't know if anyone changed my ECM programming.

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[speedracervr4]

What are your mods? You maybe able to bypass the VEC to rule that out as an issue.I had an issue with over fueling and it ended up being an injector circuit board on the VEC. Depending on if you have the same harness it's as easy as disconnecting the VEC end and reconnecting the OEM connectors. They are located behind the drivers side valve cover..

If you have heavy mods like a supercharger and bigger injectors then you will not be able to bypass the VEC without an SCT flash tune.

 

[Dan Cragin]

It sounds like a VEC issue to me.

 

[xrealm20]

How'd you get 300 miles to a tank in the viper? I don't think I've gotten that much before

 

[Tom and Vipers]

Yeah, it was the VEC1 allright! When I suspected bad piercing connectors (installed around 1998), I decided to solder all the connectors at the ECU.

The MAP signal wire which is cut and the VEC1 is "inserted," one of the crimp connectors FELL OFF THE WIRE!

Over half the strands of the wire had corroded to dust - I think there were only 3 or 4 strands left and they were much smaller than their original gauge.

So I solder those connections and the piercing connection for the O2 sensor.

Oh and of course, the MAP wire was cut 3/4" from the connector which left me only 1/2" to work with to solder the joint. Rrrrrr....

But wow, when done, rock stable behavior like I remember when I bot this car (Skiles' BadViper).

The 3000 rpm idle hang is gone (if a little hang [~1400] shows up, just a click or two more positive on the VEC1 LO - the VEC1 is set +4 on both LO and HI.

But the fuel economy on the interstate definitely will achieve 300 miles per tank easily (have gotten 250 with 1/4 tank left but that last bit was NOT highway miles (!) I believe 22-25 mpg will happen. And this is at 4500' altitude. With the VEC1, can lean out and advance timing on the LO which shouldn't hurt the engine I figure. I never go negative, just back to zero. Can tell it gets real flat at that setting.

Oh, the piercing connections: I exposed wire carefully by cutting insulation with razor, wrapped VEC1 wire around the exposed wire, and now a T-joint, I built it up with brush on vinyl coating - the type for dipping tool handles.

I have pics of all this but unless I host them on 'Bucket, won't be able to insert them.

 

[Bruce]

I'd like to see the pics.