1988 F350 with smog control issue

Propane, Butane, LPG, GPL, C3H8, C4H10
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WillieRides57
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Joined: Sat Aug 30, 2014 2:34 pm

1988 F350 with smog control issue

Post by WillieRides57 »

I am new to the forum and am having trouble with a 1988 Ford truck w/ 5.8 Windsor and manual tranny that I am working on. It has a IMPCo 425 mixer with associated conversion equipment and has 62,000 on the odometer. The truck runs well but with a slight miss and needs a California Smog certificate. When first tested, emissions was VERY high on NOx (4,100), low on CO% (.02) and high HC's (170 ppm) on the 15 mph dyno test, then 3.500-NOx/.02%-CO/176 ppm-HC on 25 mph test, not meeting specs. This truck never had any work done except oil changes and the battery probably was replaced a few times. Even though I don't really know propane IC engine systems, like any engine I figured to start with the basics. I went through the ignition system, found a bad cap, rotor and plug wire but replaced all the plugs and wires because of age. Replaced cap, rotor and did the O2 sensor also. I also found the EGR valve was sticking and got it freed-up and headed back for a re-test.

The re-test resulted in improved NOx (down to 2,200), CO% still around .03% and HC's at 156 ppm; still not good enough to pass. Now is where I started scratching my head because in my mind the only thing left to do is replace the CAT. Any ideas?? I know there are different profiles of gas valve for this mixer but am so close to the "allowable" limit that it seems attainable. I thought overnight about the air pump but checked it today and it is working fine. Am I missing something? Should I try backing the timing down a bit? Any help or pointers would be appreciated.

Bill

franz
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Re: 1988 F350 with smog control issue

Post by franz »

High NOx is created by running excessively lean. The HC isnt too bad for an open loop engine (no computer controls it looks like). There are four gas valves for the Impco 424, the one most likely for yours is the #16, the standard open loop valve. The #44 is pretty lean at cruise and the #51 is rich for closed loop. The number is cast into the bottom of the cone of the gas valve. It is also on the datatag on top of the diaphragm reinforcing plate inside the mixer.

The giveaway here is the low CO and relatively low HC. Take the gas valve out and clean it. Make sure it moves freely and the diaphragm isnt hardened. The little diaphragm on the side is paper thin and tears easily. It controls the idle and low speed mixture.

Franz

storm
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Re: 1988 F350 with smog control issue

Post by storm »

franz wrote:High NOx is created by running excessively lean. The HC isnt too bad for an open loop engine (no computer controls it looks like).
I can't comment on USA vehicles but certainly in Australia that vehicle would be have an ECU so I would assume the US equivalent (and more specifically the Californian model) would also have an ECU (Fords EEC-IV). If this is the case (and it would certainly be in Australia) I would wonder why it hasn't been fitted with an stoich controller (@OP, see attachment) of some sort. It would be mandatory here in Australia (even though customers do remove them after fitting).
Attachments
CP201-Impco-processor.jpg
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

C3H8
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Re: 1988 F350 with smog control issue

Post by C3H8 »

Storm, your correct. Normally a California conversion would have a controller on it but were dealing with a 26 year old truck here now. If it had one it was possibly removed.

Franz is pointing in the right direction. To reduce Nox, HC and CO with a cat you need the correct amount of fuel. The high Nox definitely points to a fuel mixture that's to lean. The best method is with an analyzer that shows fuel air ratio. For a cat to work properly the F/A ratio has to be stoich or 15.5:1 on propane. Another method would be to use a scanner or volt meter to check the O2 reading. It should be bouncing back and forth across the 450 MV threshold at those 15 and 25 MPH test speeds. Besides the mixer I would do a thorough check for intake manifold vacuum leaks also. These early model injected fords were notorious for developing manifold leaks.

C3H8
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Re: 1988 F350 with smog control issue

Post by C3H8 »

I should have also mentioned that replacing the O2 would have no bearing on your propane fuel mixtures unless you are using a fuel controller for the propane system. The O2 would be capable of showing the current approximate mixtures but it would not be able to change the mixtures if it was an open loop system.

franz
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Re: 1988 F350 with smog control issue

Post by franz »

Even though conventional fuel systems were closed loop in the 80's, closed loop wasnt required on alt fuel vehicles until around 1992, based on the assumption that the alt fuel was cleaner. There were several companies making closed loop controllers for these early applications and many of us adapted the OEM computer to control the alt fuel too. Not easy but it worked, sort of.

I rarely saw a closed loop controller prior to 1992 and MANY, repeated, MANY were disconnected, and many more were installed but never functional!

WillieRides57
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Re: 1988 F350 with smog control issue

Post by WillieRides57 »

Thanks for all the responses. I should have prefaced my original post by stating that the NOx on the truck is now within spec, just trying to move the HC's closer to <130 PPM. I am pulling the mixer off and having a look. If it was Holley 4150 or 4165-well it would be easy! I am checking the timing too, static and vacuum advance. Again thanks for all the tips.

WillieRides57
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Joined: Sat Aug 30, 2014 2:34 pm

Re: 1988 F350 with smog control issue

Post by WillieRides57 »

This is the truck that keeps on giving: headaches, that is. After checking for vacuum leaks, timing, compression etc. determined replacing the catalytic converter as last option. What a strange thing to find that the front 1/2 of the CAT honeycomb is missing? Where did it go? I am sure this is a Ford factory unit since this truck only has 62,000 original miles. It gets better.

After welding in the new CAT and reconnecting things I crank it up and wow, what a horrible idle. I monkeyed with the idle control a bit but still seems rough. Funny, the engine is very smooth over 1,500 RPM so went back to the local smog station. The technician has been kind enough to re-test through this whole process. The results were not what I expected. A real slap in the face was seeing the HC's go up to over 800 ppm. I am at my wit's end here. I kept the old CAT and am wondering if I have a back pressure issue or something is wrong with the EGR system,in regards to the IMPCO propane system. The air pump is pushing a lot of volume and the EVP diaphragm is working. I back probed the voltage and EVP unit is within specs. I am stumped.

jono
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Re: 1988 F350 with smog control issue

Post by jono »

I am not about to resolve your issue, but, you did mention it has a miss ! A miss, as in not firing correctly or at all would pump out some bad air richness ?

For some to consider the concept -a very tidy lean bias logic ( LBL) system has been developed in Australia I believe for our market.Uses IMPCO fresh air mixers with a leaning ring or two, with enrichment coming from the port plug on the converter where we also bleed off for a primer.I'm not certain whether its TPS requirement is a simple Throttle Position Switch on, off, wide open function or a variable resistor sensor readout. Seen some awesome stoichs coming from these units, that are an opposite to the older pollution controllers that, yeah, always seem to be disconnected, lousy plier crimp connections, and or in som level of distress. A set and forget sytem the old things were :(

www.peelinstruments.com.au

WillieRides57
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Re: 1988 F350 with smog control issue

Post by WillieRides57 »

Yes indeed, the engine had miss until I changed #3 plug wire. Since this is NOT my rig and I'm working on it as a favor for a friend, I am doing the minimum. Thanks for the advice though, really appreciate people willing to respond.

bumpstart
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Re: 1988 F350 with smog control issue

Post by bumpstart »

interesting about the LBL method of operation .
i found a similar method for myself
( modding a negative pressure feedback system ( impco 225 ) with the open loop air/gas valve
and using a manifold pressure activated power valve off the LP deck of the convertor to improve acceleration mixtures )
[ am also using a mazda boost control solenoid as the mixture control solenoid on the convertor as the impco one is not boost rated ]

this gives a lambda idle and almost a lambda cruise , but will enrich on acceleration to whatever you set the line pill orifice size for
( my electronic power valve also works as the start primer )

generally it achieves lambda a lot more often , when i am not booting it

this gives an extra dimension to the tune
where with the older system using the feedback air/gas valve
you seem to have to compromise in one area to get idle or power mixtures to spec

for the OP. without knowing to much about the ignition system ..
it seems you may be doing a bit more of internal EGR now it has more backpressure and this is quenching full combustion early

am thinking maybe just a little more gap and a little more lead in for the static timing for a more complete burn

the reason i think so.. as you describe a change i noted for myself when swapping from a wastespark system to a full sequential ignition
the wastespark definitely burned out much better and scavenging was more effective
,, and idle was much better than with the supposed upgrade to sequential coils
( however sequential coils bring much better avail dwell windows with revs )

a little more timing and a little better exposed spark over what i would have run on the old system definitely improved the idle
when the going is hard, don't retard, remember your lubrication

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