Lean engine cut on cold enrichment

[Steven A]

Hi all

Seeking some thoughts and advice.

After much fun and games, have the cold and warm up enrichment almost working fine.     Until temperature gets to spot on 180 deg F on the instrument cluster gauge, all good, idle a little rich (10:1) through the warm up phase, fine under load.   In a very narrow operating temperature range, can get lean engine cut at idle.  Driveability through here is excellent, except for the old lean cut at idle  Once just over the 180 mark , all good again.   

The characteristics in this narrow temperature band is it comes down nicely to idle, thinks about it 20 or so seconds at say 11:1 Air/Fuel ratio, then leans off - once it gets past 14:1 that is about it and rapidly goes lean cut.  Slightly cooler or very slightly higher operating temperatures, if it does lean out a little, stabilities and comes back to around 11:1 which is about where the idle is with normal operating temperatures. 

Having already adjusted the WRD via shim height, don't really want to further change-here as so close to where I want to be, feel that I missing something here!

A/F ratios are from A/F sensor in the exhaust downpipe - fascinating to watch/see how the mechanical fuel injection systems copes with driver demand, torque back up, complex relay controlled CSV and Fuel Injection pump solenoid etc.   

Appreciate any advice

With thanks

Steven

[Tyler S]

Steven, Did you add or remove shims under the WRD? Reason I ask is this also affects the warm up regulators air intake to rack position calibration. If you added shims it will richen the injection pump during warm-up cycle but the same amount of air will still be drawn from the WRD intake. This will cause rich warmup. Opposite is true. If you removed shims under the WRD, you are leaning the IP. The warmup cycle will be lean because you still have the same amount of air v.s. temperature from the WRD. During this "Hiccup", check that the air filter on the side of the WRD is not sucking any air. If it is you may need to remove the WRD thermostat and add small round shim/shims to the top of the air valve. If you do this you can add the same thickness oval shims under the WRD to compensate for the now longer WRD pin. I hope this makes sense. On another note, I fought with this same thing. I ended up putting everything back how I found it and used the round shims under the barometric compensator to adjust the A/F. It ties into the same lever in the IP. It's easier, a lot more forgiving, and wont mess with your warm-up cycle.
Tyler

[wwheeler]

My understanding is that the small round shims move the position of the WRD piston in the bore and therefore changes the air AND fuel volume. In theory, round shims don't change the cold running mixture. In contrast, the large oval shims move the position of the pin in the pump and only effect the amount of fuel being delivered which changes the mixture. In theory, you shouldn't have to adjust with oval shims when changing round shims. I say in theory because it isn't perfect. 

Changing round shims on the air piston changes the amount time and temperature that the warm cycle is active. Too few shims on the piston and it may not ever shut off (sucking air from the filter port). Too many shims and the WRD barely comes on unless at very cold temperatures.

At least that is my understanding.

[Tyler S]

Wallace, changing the small round shims or larger oval shims both have an effect on where (how long the pin is) during warmup at a certain temperature ranges. Adding small round shims will not only close the air flow sooner, the pin will be further down into the rack, leaning it out. Adding large oval shims will richen the mixture but not compensate the piston for air in the WRD because it is "down the line". Likewise removing large oval shims will lean the rack out but have no effect on the timing/temperature of the air flow. Its a balancing act.

I think a good analogy that everyone could understand is a push button ball point pen.
You have the tip of a retracted pen against a piece of cardboard with a hole in it. You slowly push the button a certain distance and the tip of the pen protrudes through the hole a certain distance. Your thumb is the WRD thermostat. You now add a small round shim between your thumb (thermostat) and the pen button (WRD air valve). This extends the pen tip further down into the hole. You now add a large oval shim the same thickness between the pen tip case and the cardboard (WRD housing and FIP). This brings the pen tip back up the hole to the same level as when you first started. But now the pen case (WRD housing) is in a different place, which is where the air ports are. The pen is your WRD, with spring, piston, case etc. The end question is what do you want the tip (rack pin) and button (WRD air valve) to do when.

These shims were never intended to be used to adjust your rack during operating temperature. This can throw off your warm up cycle. They are used to make up differences within the WRD itself, as well as the thermostat. All thermostats will have slightly different characteristics. They are just a wax plug behind a piston. Pretty inaccurate comparatively. Adjusting the rack should be done with the rack screw behind the small allen hex bolt at the rear of the FIP. This is how Bosch intended it. But yes it can be a pain to get to so we all look for an easier out.

[Steven A]

Thanks to all very much for responses, appreciated,   It certainly helps my understanding, and will work on ironing out the cold stall.

Pretty amazing the engineering from 50+ years ago to make a mechanical injection system (with no engine airflow measurement or control apart from the mechanical accelerator pedal linkage) work across the range of load/speed and environmental conditions.

Will report after next round of fine tuning

Thanks

Steven

[wwheeler]

Adding small round shims will not only close the air flow sooner, the pin will be further down into the rack, leaning it out. 

These shims were never intended to be used to adjust your rack during operating temperature. This can throw off your warm up cycle. They are used to make up differences within the WRD itself, as well as the thermostat. All thermostats will have slightly different characteristics. They are just a wax plug behind a piston. Pretty inaccurate comparatively. Adjusting the rack should be done with the rack screw behind the small allen hex bolt at the rear of the FIP. This is how Bosch intended it. But yes it can be a pain to get to so we all look for an easier out.

With round shims, I am not sure I see how if the air volume is being reduced along with the pin reducing the fuel, that it would change the mixture. I have taken these apart many times and have seen that as the piston slides down is closes the opening that lets additional air in along with the pin reducing the fuel.

I totally get the oval shims and yes they change the mixture but ONLY at cold running temperatures. The oval shims have little effect when fully warm. There have been numerous posts in here over the past years about the "stop" post in the pump. Once the WRD pin hits that, the mixture will not change further. The spring in the WRD takes up the expansion when the wax bulb continues to grow in length from increasing temperature. Eventually, the spring force and the expansion force from the wax bulb reach an equilibrium and it all comes to a stop. 

   

[Tyler S]

Wheeler, Yes, you are right about that. However the fip will be leaned out too soon in the warm up cycle causing a lean condition if not fully warmed up. The reason I explained it that way was because of the OP complaint. Didnt want to confuse him. i have seen on more than one occasion that the thermostat maxes out but does not fully close the air valve. So you add round shims to compensate. Then you add oval shims to bring the pin height back. The WRD setup bottoms out well before the FIP lever "stop" will bottom out so yes changing the oval shims at operating temperature WILL have an effect on FIP setting. The Baro sensor acts on this same lever and you can shim it in the same way at operating temp to change fuel delivery.

[ja17]

Yes, the shims are basically factory settings. However, back then the factory did not consider wear. Once wear begins the ratios change. These pumps tend to wear rich during the warm up cycle, so oval shim adjustment is a logical choice for adjusting rich cold running situations. Often times it is the most effective choice for rich cold running, since rack adjustments change mixture at all temperature ranges and all engine rpms at once. I think you are right Tyler on the round shims. Changes here do not change the dimension between the air slide location and the end of the pin. Rather both would move together keeping the relationship between the two the same. In this respect having the air slide close off the extra air supply at the right temperature is ground zero. Much deviation here can throw a lot of things out of wack.

[wwheeler]

I guess I am a little confused about the function of the round shims.   I understand the function of the oval shims. I understand that the lack of round shims can cause the additional air not to be ever to be shut off (air valve not closing off the port in the housing).

But does changing the thickness or number of the round shims effect the fuel mixture when the engine is cold?     

[Tyler S]

Wallace, Yes removing round shims will effectively start the FIP lever in a richer position when cold. And end up in a richer position when at operating temperature. The effective goal is to use the round shims to have the air valve close completely just before operating temperature. Then go and adjust the oval shims to get the air to fuel ratio correct during this cycle.  Oval shims ONLY adjusts the fip rack lever position.  Round shims affect WRD air flow AND rack lever positions under the various temperatures.

Joe, yes that is exactly the point I was trying to express. Not to use the oval shims as an adjustment for operating temperature a/f adjustments. The rich wear in the system is most likely caused by the WRD. All these contact points wear and that allows the FIP lever to move "up" to a richer position. The thermostat itself can get lazy. Not shutting off the air valve completely and again causing the FIP lever to stay in a rich position.

One more note. When I worked for Nissan we had a similar setup in the Z24i engine used on the old hardbody trucks and 240 sx. An almost identical looking wax core thermostat. These were problematic with some of the other techs. One thing to remember is these have no return spring built into them. If you heat them up with no tension on them and then let them cool you ruin the thermostat. It will draw air into the chamber. Never pull on the stem or take one out of a hot motor and let it cool on a bench. If you bought a new one make sure it came with a shipping retention clip installed on it! Nissan even released a TSB on this very issue.