swoop,
the FCSV that only rattles when the dwell is not 0 or 100%, since my dwell pegs to 100% after 30 seconds of running, the FCSV could not possibly be rattling.

when the engine is running does the fuel control solenoid start clicking? (sounds like a rattle almost but it is just clicking real fast)

if it is not doing this then that is the problem

Edward,
Finally got back to the job-site and no go on the vacuum elbows pushing on the diaphragm. Still stuck in the same boat.

thanks, you just gave me the clue that may fix your truck.
The elbow that screws into the regulator cover and connects the FCSV with vac hoses to the regulator, should be only screwed in about 4 threads, and remain ONLY finger tight.
The threads on this elbow, if screwed in 'normally tight' will hit the metal part of the diaphragm internally in the regulator and prevent proper motion of the diaphragm.

The supply at the mixer is very limited, just trying to get the point across that every drop of vacuum that is being supplied to the FCSV doesn't lean the mixture out enough to get the O2 volts to drop at all. Disconnecting the wires to the FCSV does nothing but make the mixture even more rich.

you do recognize that when this unit was new, the FCSV was what supplied a limited amount of vac to what had previously been the atmosphere pressure side of the regulator, and that limited amount of vac is what controls the rich or lean of this system, and as such it should not have a direct vac connection. if you disconnect the one of the wires on the FCSV, what does the o2 volts do?.

ECU has a vacuum line, the mixer that was on the truck was in poor condition so I changed it out with a brand new one I had in stock for testing. I will probably re-install the original mixer once this problem is solved. What is very strange is how fast the O2 volts climb to.9. Even if I run the vacuum hose directly to the regulator from the port on the mixer, eliminating the FCSV the O2 volts still peg at.9 There is no doubt the fuel mixture is rich.

ok thanks
one more question
does the ecu have a vacum line running to it? or no?

reason i ask is they made so many changes from pre-tier1 and tier1 , i need to know so i can figure out which system is on this. There are several service bulletins to look through. also some had a check valve in the vacum line between the converter and the intake air inlet hose some did not.
And with the mixer thing, the diaphram first came out without a shim between the bullet/shutoff button and diaphram centerplate and there was a barcode sticker on it, but they had issues exactly how you are explaining and they later came out with a revised diaphram, the newer one has the shim and no barcode sticker on it.
And so far every mixer i've looked at does have that dwell screw located under the screw journal at the base of the mixer body behind a plug. If that mixer is a aftermarket part then that may be a problem and it will not work.

AT83D00683 and no it is not dual fuel

LTRM,
i should of asked this from the begining
what is the serial number of the truck?

is this a duel fuel truck?...
(most likly not but i had to ask)

There are different part numbers that do not cross between the cobra regulator in an impco system on the af82c (pre CARB) and the af82d model cobra, (CARB tier 1, a 2003 manufactured truck of which the s15g system is, unless the truck has the s15g system that was pre-CARB for speed controlled [Home Depot]) trucks.
I can confirm that putting the wrong regulator on the truck causes bad running.
If you have confirmed positions of all vac lines as correct, you may also want to confirm no side play on the distributor shaft (up and down play is ok , but.000 free play, left to right, is a must).

I believe there is a difference for the simple fact that you can buy rebuild kits for standard model J and cobra converters, but according to cat service manuals, they have the converter for the s15g trucks as a non-serviceable part

The magic screw in the brand new mixer which is not behind the plug yet is the idle air mixture adjustment and it is adjusted full lean. I can see my dwell is 101% through the MCFA diagnostic software. My Tune up is fresh and perfect. My question remains the same!!! Is there a fundamental difference between the standard cobra and the OEM cobra supplied by Cat for the S15G fuel system. So far the only difference I can see is the cover. Impco's website is useless. The fuel control is verified as 100% functional. The only thing is is making any sense is there must be a difference in the regulators

In this system, "dwell" is not the same as "dwell" on a truck with distributor points ignition system. In this instance, "dwell' refers to the % on time time between pulses to the fuel control solenoid valves (FCSV) not the degree of time the ignition points are open.
As Mengr noted, there are differences to internal springs and gaskets between different models of "cobra" LPG regulators and they are not all the same, even if the outside looks the same, except for markings on 1 cover.
First let me run through a few checks that I consider 'first-checks' for this system.
There are 2 vac hoses on this trucks throttle body. it is a known problem that the orifice inside the throttle body can become clogged, and they are very small and fine orifice of which the size and position is important to the design of the truck, so I take a fine welding tip cleaner and run it up into the orifice to be sure no carbon buildup is restricting vac. Next it is important to be sure that the correct hose in in the correct position, so be sure to confirm that the hose that goes to the distributor vacuum advance is the hose that is closer to the mixer and away from the manifold (top of the 2 orifices). NEXT, if this unit has the computer module that when you take the round (computer cable access port) cover off, you see only plastic potting and not resistors and transistors (original), then the vac hose that runs from the throttle body to the computer module should seem rather long, and it's length is important, as the vac drop from the length of hose was included in the calculations as to how this machine reads it's 'manifold air pressure', and 30 inches is not wrong, so if you or someone else cut the hose to 'look' like the right length, then you will want to replace that hose with the correct length hose. That hose will seem long, and will hang down almost to be the lowest spot under the truck. Units where you see the resistors and transistors instead of potting material inside the computer module do not require the vac hose, and the plug on the ecm is a closed up plug (second generation ECM for these units as the first supplier went out of business).
Next (and by far the most common reason for this error actually, in my experiences) the engine tune up, so RE-tune up the engine completely including engine timing (checking with timing light, with vac line off and plugged at the distributor, then reinstall vac line and confirm distributor vac advance is working, watching the light on the timing mark to be sure it advances with rpm increases) and spark plug gap (gap really does make a difference in these trucks).
My W.A.G. off the top of my head is that you have the vac hoses on the throttle body in the wrong spots. Although it is also very easy to mistake some of the vac connections on this truck as elbows or tees when they are really valves. the elbow mounted on the air-horn over the engine valve cover is not just an elbow, and there is a tee to the vac lines near the FCSV that is not just a tee, and an improper connection between any of those hoses can make these trucks run terrible or not at all.
When ever teaching anyone about s15g fuel control systems, everyone should be warned not to test the FCSV on these trucks with anything other than the software, as there is a real possibility that testing the wires will let the magic smoke out of the ECM, and it is a pulsed signal anyway.

as i suggested
there is another adjustment that is hidden behind a small plug
standing on that side of the truck looking at the mixer if you look at the right side of the mixer you will see a small round projection in the casting at the base of the mixer bowl that will have a plug in it inside a small hole about 1/4 in in diameter. That plug has to be removed so you can get to the dwell adjustment screw.
You will need the MCFA Diagnostic software on a laptop and the cable used to connect to the ecm so you can correctly setup the system.
From your description of the symptoms this is what is wrong
turning the idle/air mixture screw will not accomplish what you need. I could be wrong but i doubt changing the converter/regulator will change anything much either. Unless it is freezing up, truck is running like crap stumbling on itself or leaking lp gas the converter/regulator is probably ok.

Up to you though...

After running the truck for only a minute, the O2 volts are pegged at.9, the dwell is 101%, the idle mixture screw is turned out to completely lean. In addition, the only adjustment is the idle air mixture with this system. I do have the software to monitor the dwell. The only time my O2 volts drop is when the engine is under full rpm at full load with a deadheaded tilt. My O2 volts drop to.2 and the dwell creeps down. As the dwell creeps down the O2 volts creep up closer to the.5 that is desired. Everything is pointing to the LP regulator. I cannot seem to get the info on the OEM cobra that came with the truck compared to the Cobra that is currently installed. Cat wants $200.00 for the OEM lp regulator. Before I buy it I would like to know there is a difference other than the cover.

Thank You everyone for your input so far.

LTRM,
gee seeing what you said about all the problems with this truck before you started i was reluctant to even say anything here, that truck sounds like its a wreck... but....i may be able to help a little bit...

this is an excerpt right out of the troubleshooting HPK doc from MCFA about troubleshooting the S15G system. Naturally i cannot post the whole document because of my position but i can give you this bit of info that may assist you in some way.

Test the oxygen sensor:
a. Observe the oxygen sensor voltage, 0.2 to 0.9 volts is the desired range.
If the oxygen sensor voltage is 0.9V or above, the dash light will come on
indicating a system problem. The error code will be for the oxygen sensor.
This is normally caused by an incorrect dwell reading with the resulting
problem being in LP mixer or the throttle body.
NOTE: Rarely is the oxygen sensor the problem.


Do you have the means to run MCFA Diagnostic program?
If so you may need to do this... if this is a OEM mixer then the dwell screw might not be readily available and you will have to remove the plug they put in place over it. Once that is removed a adjusting tool can be inserted into the hole to the adjusting screw to make the adjustment. DO not attempt to adjust this without using the Diagnostic tool or you will fubar it up worse. Keep in mind though that the plug is in place for a reason, these are pre-set during production and are only intended to be adjusted by qualified personel. ;o)

since you said you put a new mixer on i thought this may come in handy. The ideal dwell angle the S15G system runs at is between 35-45 % at idle and 80-100% at full RPM
You also may want to double check the throttle body.

I believe the new mixer diaphram part number that addresses some issues with the earlier version mixer is SE000071. Being a new mixer it should already have this updated diaphram in it.

I believe there is a lean mixer diaphram, not sure the part number.

LTRM;
Use a length of 5/16 dia. clear plastic tubing bent in the form of a U tube with a tape measure sandwiched in the center and you now have a water column guage. N. Am. is an abbreviation for North American market (USA, Canada and Mexico), while EC the standing abbreviation for European Common market built trucks.
I would hesitate trying to use the power adjustment mixture to compensate for incorrect metering orfices and secondary spring in a vaporizer - regulator because of balance over a wide operating range. Main LPG components between the markets are engineered to match and compliment each other. With the addition of engine management systems for large spark ignited engines in the US market falling under EPA regulation one could find one's self in a gray area that could lead to some healthy fines. Keep in mind the way the wording of the regulations are that any changes regarding safety must have written approval from the manufacturer. The UL label is what you are in conflict with also.

The mixer on this tier one system only has an idle air fuel mixture adjustment. The screw is set as lean as possible. In addition, the mixer is new.