Fuel Cooler using AC lines??

[sigchuck]

Hi guys, I regularly drive my 930'ed 911SC year round, including in heavy traffic with 95+ degree temps. Last summer I noticed that on very hot days during stop and go driving, my AFRs at idle would creep up slowly throughout the course of a drive to around 16.5 or so.

The CIS system is in tip top shape, I have a relatively new single -044 pump, and AFRs are normal during driving and come back down at idle if I apply a little throttle (say go from 1000 RPMS to 1200).

It seems that the likely culprit is warm gas, and in researching the issue a bit, it's likely exacerbated by the high volume pump I have (as well as the hot asphalt and all the fuel being returned to the tank at idle).

It looks like Bosch has developed a fuel chiller for some makes using CIS, and that Porsche used one on A/C equipped versions of the 928. It seems like it just routes the return line through a valve that's in series with the AC lines coming out of the evaporator and heading back to the compressor.

Has anyone ever run into this issue and had it bother them enough to attempt to cool the fuel, maybe even adapting the 928 cooler to the 930 CIS?

[Toddrivers]

I have no direct experience with cooling fuel other than running the fuel line thru a can of ice at the race track.
In the summer I always need to adjust my WUR to the rich side to compensate for the higher temps so having an adjustable WUR is a necessity for me. The first time I drove the car in 100* West Texas temps I realized I needed two things, a fender oil cooler and an Air conditioner. After 10 years all is good.

[flightlead404]

I've had this "creeping AFR at idle" problem myself.

It turned out to be an indication issue, not actually an AFR issue.

I'd be surprised if your car idled at all at 16.5, it would certainly be very poor.

Are you positive this isn't an indication issue? Does the quality of the idle change as the AFR changes?

[sigchuck]

Idle quality may drop off a bit, but nothing concerning. It's been pretty temperate for the past few months, but I do recall instances of maybe losing 100 RPMs or so. These are also days where I'm running the AC, which will also cause a slight drop in idle, so maybe my recollection isn't as good as I thought.

I should have also mentioned that oil temps are great during all of this and max out at just a hair past the first white mark.

That does make me think though that maybe it's a chicken/egg issue since AFRs return to reasonable range if I increase idle. I do have a spare distributor that I need to send out for rebuild as I'm pretty certain mine hasn't ever been touched, notwithstanding a split case rebuild and rebuilt CIS. It's always bothered me that it's the potential weak link.

[flightlead404]

I've driven my 930 in temps well into the high 90's in stop and go and never had issues with hot fuel causing rising AFRs.

Oil temp for sure before I put in the thermostatic oil cooler fan, but it never impacted AFRs

I'd make sure there isn't something else before chasing that. ymmv

[sigchuck]

Any other thoughts on what else it might be? I'm definitely not in a hurry to open up my A/C system again and introduce a part that's not supposed to be there into that system or the fuel system, but I am at a bit of a loss.

I've tried two WUR rebuilt by reputable sources (one being Flowtech), so I don't know that would be the weak link. I really don't think the lower idle is causing lean condition (I'm thinking vice versa), but plan to install a rebuilt distributor anyway as I have no rebuild history on the current one.

As mentioned above, the engine oil temp is definitely okay, and while I had initially thought hot air might be the culprit, that doesn't make much sense since it's less dense and I think would cause a rich reading with CIS.

The single -044 pump is pretty high volume, but I did a fuel return test when I was chasing what turned out to be a non-existent issue, and the return volume was within spec for the dual pump set up, but definitely not off the charts high - really wasn't high at all.

Some other thoughts:

- Indicator Issue - I have the o2 sensor downstream of the turbo - could the turbo be acting similar to a catalytic converter sitting in traffic on a hot day, thus skewing the reading? Even with headers, the K27 I have (probably way too much turbo) doesn't do it's thing in stop and go traffic since there's just not much opportunity to get into the throttle. If it's not making boost, is it really going to get that hot? The slight drop in idle I'm experiencing also doesn't really gibe with it being just an indicator issue.

- Tank venting - most of my original evap system is in place except for the charcoal cannister in the engine bay, though the line that runs to it is in place, just not hooked up to anything. I haven't really checked this system out, and don't recall ever having any hissing when I take off the gas filler lid, but could a clogged vent lead to hot fuel issues?

- Replacement tank - I replaced my original tank with a Dansk tank a few years ago. My recollection is that these didn't have the same swirl pot design as the factory tank, and I'm not sure where the return might hit and what difference that would make to fuel temps. I would think that returning to a swirl pot where the pickup is might potentially exacerbate a hot fuel issue (returned fuel heated then immediately put back into circulation before mixing with cooler fuel in the tank), but...

- Single, high volume fuel pump - would a single pump vs the 930 dual pump design have the potential to heat the fuel enough to make a difference? Maybe not in normal operation, but on a really hot day?

[Toddrivers]

I really doubt fuel temp would cause problems and any effects of vapor lock in fuel systems went away with high pressure electric fuel pumps.
I have a fuel pressure gauge in the console along with my AF gauge which allows me to see how fuel pressure affects AF ratio. I always have to adjust the WUR in warm weather to maintain 13.5 -14 AFR (53-56 psi fuel pressure).

[sigchuck]

Thanks, Todd - I would have thought so, but the fact that a lot of manufacturers used coolers with FI and electric pumps (Porsche with 928, Jaguar with quite a few models) is what made me wonder. I don’t think I’m getting vapor lock as much as I’m maybe getting slightly less volatile hot fuel.

What were your observations in summer? Was pressure increasing out of spec and causing a lean condition? My initial thought last summer was that the WUR was maybe getting too hot, the spring flexing too much, and thus causing too high of control pressure resulting in a lean condition. I could confirm with gauges I guess but the idea of cracking pressurized fuel system on very hot engine was a little off putting.

[flightlead404]

Originally Posted by sigchuck

Thanks, Todd - I would have thought so, but the fact that a lot of manufacturers used coolers with FI and electric pumps (Porsche with 928, Jaguar with quite a few models) is what made me wonder. I don’t think I’m getting vapor lock as much as I’m maybe getting slightly less volatile hot fuel.

What were your observations in summer? Was pressure increasing out of spec and causing a lean condition? My initial thought last summer was that the WUR was maybe getting too hot, the spring flexing too much, and thus causing too high of control pressure resulting in a lean condition. I could confirm with gauges I guess but the idea of cracking pressurized fuel system on very hot engine was a little off putting.

Until you have fuel pressure data all you can do is guess. It seems to me right now you are grabbing at straws and talking about WUR problems that I've never heard of. Excessively flexible springs due to fuel temps? Sorry man, not buying it.

You need to validate for sure that your AFRs are indeed changing, and its not just an indication issue. Also remember, excessively rich conditions can show as excessively lean conditions on a WBO2. You need solid fuel pressure data as well.

Its getting warmer, you should be able to duplicate this problem at will in another couple months. Get a CIS pressure testing kit, learn how to use it.

[sigchuck]

Thanks - I have the kit and have used it quite a bit. I’m talking about the spring in the wur that heats with electrical current getting heated even more but hot engine compartment air when it’s warm out, thus leaning out the mixture past spec.

Both my WUR check out doing the typical WCP test, which is generally done on a cold engine with 12v applied to the WUR. My hesitancy was with working on a 200 degree engine (problem only presents when car is actually warm, not just doing wur test). I’ve not had much luck with not spilling some fuel during the tests. If the engine is hot, lines fully pressurized (because I just shut off the car), it seems like there’s potential for some fire. Maybe not, but definitely some burned hands on the engine.

I’ve plumbed in a permanent gauge previously, so I may get those connections back out.

[Toddrivers]

Originally Posted by sigchuck

Thanks, Todd - I would have thought so, but the fact that a lot of manufacturers used coolers with FI and electric pumps (Porsche with 928, Jaguar with quite a few models) is what made me wonder. I don’t think I’m getting vapor lock as much as I’m maybe getting slightly less volatile hot fuel.

What were your observations in summer? Was pressure increasing out of spec and causing a lean condition? My initial thought last summer was that the WUR was maybe getting too hot, the spring flexing too much, and thus causing too high of control pressure resulting in a lean condition. I could confirm with gauges I guess but the idea of cracking pressurized fuel system on very hot engine was a little off putting.

Several Years ago Brian Leake rebuilt my WUR to make it adjustable, I then added a sender to tap control Pressure from the WUR to a gauge in the dash, added the Innovate LM2 Air Fuel monitor and gauge next to the CP gauge. I have found to keep afr and cp in spec year around an adjustment at the bottom of the wur to lower the cp (richer) in the summer and raise the cp (leaner) in the winter is necessary.

[sigchuck]

It was a beautiful 74 degrees on my way home today, definitely not hot, but I did start getting higher than normal idle AFR readings.

I plumbed in a fuel pressure gauge and it was solid at 48 psi both this morning (temps in high 40s) and this evening after completing the warm up phase.

As stated above, I’ve had zero issues with engine temp and it typically warms up to the first white hash mark and maybe goes over a needle width or two on very hot days in stop and go traffic with AC running.

Idle AFRs were fine (14.5) with engine initially at warm temp (first hash mark) but slowly crept up at stop lights after 25-30 minutes of driving. AFRs while driving were fine and typical.

Could this just be a case of setting idle AFRs without the engine fully warmed, notwithstanding the fact oil temp had reached 180? I ended up adjusting the idle mixture back down to 14-14.5, so I’ll check in the morning how rich it is on initial warm up.

[flightlead404]

Originally Posted by sigchuck

It was a beautiful 74 degrees on my way home today, definitely not hot, but I did start getting higher than normal idle AFR readings.

I plumbed in a fuel pressure gauge and it was solid at 48 psi both this morning (temps in high 40s) and this evening after completing the warm up phase.

As stated above, I’ve had zero issues with engine temp and it typically warms up to the first white hash mark and maybe goes over a needle width or two on very hot days in stop and go traffic with AC running.

Idle AFRs were fine (14.5) with engine initially at warm temp (first hash mark) but slowly crept up at stop lights after 25-30 minutes of driving. AFRs while driving were fine and typical.

Could this just be a case of setting idle AFRs without the engine fully warmed, notwithstanding the fact oil temp had reached 180? I ended up adjusting the idle mixture back down to 14-14.5, so I’ll check in the morning how rich it is on initial warm up.

I think you have something else going on.

I set my idle mixture when the engine is good and hot, like over the first hash mark on the oil temp gauge. I'm around 12.8-13.0. Much leaner than that and I get lean pops, but seems like everyone's engine is different.

That idle mixture has nothing to do with off-idle mixture though. Once you're off idle the mixture is set by the control pressure (and the fixed shape of the cone in the air flow sensor).

[reclino]

Big V8 CIS Mercedes used these AC coolers for the fuel. I am guessing that with a low tank, hot weather the fuel begins to warm up. Warmer fuel is more likely to cavitate on the Inlet side of the pump, this could drastically reduce fuel pump life and may effect fuel pressure. Looking at outlet of the 930 fuel tank running through the built in strainer and then into the suction side of the first pump it's amazing it works as well as it does.
Reclino

[mark houghton]

Originally Posted by reclino

Big V8 CIS Mercedes used these AC coolers for the fuel. I am guessing that with a low tank, hot weather the fuel begins to warm up. Warmer fuel is more likely to cavitate on the Inlet side of the pump, this could drastically reduce fuel pump life and may effect fuel pressure. Looking at outlet of the 930 fuel tank running through the built in strainer and then into the suction side of the first pump it's amazing it works as well as it does.
Reclino

Interesting thread. Ponder this: If indeed hot fuel begins to vaporize and cause pump cavitation, and perhaps reaults in lower fuel pressure as the result, then with CIS one might expect a richer mixture: lower system pressure allowing the air flow metering arm to deflect further - opening up the fuel delivery ports more. But then fuel 'volume' may be a larger factor than pressure.
Just thinking aloud doing a brain dump. Most of my driving is in the heat of the summer, 90°F+, but I no longer have and AFR guage on board. Sometimes too much information is.....too much.

[Kiddwayaa]

Consider installing a fuel cooler or adapting the 928's fuel chiller to your 911SC. This should help mitigate the warm fuel issue and stabilize AFRs at idle.

[gorskined]

If you are running a bosch 044 chances are its counterfeit. i originally installed 2 new 044s ran great. But i had intermittent fuel issues after a few hundred miles . everything was new or rebuilt and fully adjustable . i finally installed a fuel system pressure cockpit gauge and a afr gauge. to monitor things real time while driving.
What i noticed at an idle while hot i would loose system pressure ,dropping into the low 80psi range and if i blipped the throttle the pressure would rebound back to the low to mid 90s as this was happening i could watch the arf # rise and fall . (12.5v vs 14.5 volts on a hot counterfeit 044pump ) . i replaced my two o44 with a single new bosch 200 series pump. This solved my problem, more than enough fuel on and off boost. i still see a pressure difference between 12.5v and 14.5 volts but its only a few psi not enough to effect anything .. All new Bosch pumps you need to register with bosch to confirm they are original bosch and not counterfeit pumps.

[sigchuck]

Thanks to the scammer for bumping this - spring is upon us in Dallas!

I think I’ve settled on this being a heat soaked cast iron fuel head issue and maybe a tank full of hot gas issue. Almost like starting a heat soaked motor that’s sat and dealing with some slight vapor lock like symptoms, once I get into the pedal in the slightest, AFRs are back to normal. On throttle AFRs were always great, just idle that crept up in the heat (which I’m sure the AC tail condenser doesn’t help as I’m running AC also).

Problem fixed by seasonal idle mixture adjustment. Kind of annoying, but it’s not EFI.