Weird AFR issue
 

My 930 starts right up in the morning, high idle, rich mixture, levels out to a perfect 14.7 and 950 idle. I'm happy with the AFR curve throughout the rev band. I can go for a long ride, or run stop/start errands, and everything is just great.

If I let the car sit for several hours (at work for example) and start it up (starts fine), the idle AFRs lean way out, sometimes as high as 17. Cruising is fine, boost is fine, come to a stop and idle or trail the throttle, and they jump back up. If I throttle it up to 1500 RPMs or so while sitting they come back down into the 14s, but let it go back to idle and 16-17 again.

It will behave like this until it sits overnight and then it's back to perfect again.

I'm 100% sure this is not a gauge issue, because the engine idles like crap when it's above 16 and stumbles off idle as I apply throttle.

This started about 3 weeks ago after no work on the car at all and it has me stumped. I can't come up with any logical cause. I've pinched the AAR hose when it's happening and it has no effect. I have an adjustable WUR, but I don't want to touch it since it runs like a champ after it sits all night. Thoughts? Sticky decel valve?

Thanks,
Steve

Steve,

Did this start happening with warmer weather in your area?

Every summer, I have to adjust my WUR until the ambient temperature stays roughly the same. What you are reporting is common occurrence on my car with warmer weather.

Perhaps someone else has a reason for the AFR change, but my experience tells me is has to do with ambient temperature changes on the WUR.

Adjust your idle AFR down to 13.5:1
They idle alot smoother and better there and with todays E10 oxygenated gas 14.7:1 is too lean.

The combination of a summertime heat soaked WUR after parking a while is probably raising control pressure higher than it ever gets in the winter which leans it out, and E 10 gas has you running too lean at idle.

That, or what Jim already mentioned. I may be wrong, but your decel valve shouldn't open without a a pretty substantial vacuum (as in de-accelerating with throttle closed). I'm not sure that it'll open just at idle.

In the last 3 weeks, have you seen ambient temps go up quite a bit, or changed gas supply to higher ethanol?

Since you seem to have the sequence of events pretty much down as to when this happens, try throwing an ice pack on the WUR the next time your AFR's are at 17, see if you can get them to come back down. If they come down, that should at least answer the heat soak possibility or the condition of your WUR.

All are logical ideas except for the fact that if it doesn't sit for a few hours the issue doesnt happen. That's what has me so baffled, I can start stop for hours at full temp and it's a-ok. Also in Iowa they do not put ethanol in premium during the summer.

I like the idea of ice on the wur. Perhaps full hot and full cold is fine but luke warm is causing a sticky pin. Something I hadn't had a chance to do until tonight is get it good and hot after work. I took an extra long ride home and saw the same behavior for the first 20 minutes but by the time I got home I was back to normal at idle.

That's really the delta lately, driving it 10 minutes to work, sit for 4-8 hours in the sun, then 10 minutes home or to lunch. Until recently this season I was only using it on weekends for drives or errands.

JF 14.7 at idle actually works quite nice on my build. I get high 13 on steady cruise and 11 on boost raising to 12 at redline with .8 bar. It took a lot of tinkering with the wur to get there. Ideally I think I need to get an rpm clamp to delay but increase boost enrichment. When I was in the 10s on boost I seemed to have more kick up top but the midrange was a mess, as you well know. But I digress... :)

Has anyone left the Bosch pressure gauge on while driving? Seems like it would be ok but I'm a freak about possible fuel leaks since seeing a guy's 88 Esprit go up in flames. I'd like to see some pressure data while the problem is happening.

As I've been typing I'm starting to wonder if it's simply a warm up issue. Maybe I have a weak injector or bad spark plug. The other difference is I let the car warm up before leaving for work but get in and go after. How rough are these engines on 5 cylinders? I don't notice it running rough on throttle during these episodes but the idle is definitely weak. Ah, now I'm all over the place :) thanks for helping me think this through.

Without having to tell ya Steve, these cars can sure be a love/hate relationship at times. I was wrestling with a weird AFR condition last summer, but mine was reverse of yours and only showed it's ugly head when I shut her down hot for maybe 15 minutes or so (enough to heat sink things), and my AFR's would go way rich (mid-12's) but only after I hit beyond 2500 rpms no-load. Like a light switch BOINK....AFR's would drop from normal high 13's to low 12's. I happened to notice some other weird feces going on at the same time, so hooked up a temporary volt meter in the cockpit. Like magic, when my AFR's went bannanas the voltage also went to something like 16. Yowsa, I said, must be my voltage regulator crapping out and causing issues with my Innovate AFR setup.

New regulator is sitting on the shelf...to be installed in the next couple of weeks.

Just think, if you didn't have the ability to monitor your AFR's you would just go on fat and happy. In my case there isn't a damn thing wrong with my fueling, just with the fueling monitoring electronics (or so I postulate, anyway).

I'm with Jim, I too had a similar problem, and when I lowered my "idle" AFR's to the 13's, it went away...

Unfortunately I think it's a flaky injector. Today when it was acting up I found the #4 header pipe was running 150 degrees cooler than the rest. Since bad spark or compression would show up rich I think the injector might be the culprit. It will be a while before I can swap injectors so she's going to sit.

I'll try fattening the mix but in not hopeful since mixtures are great when it's dead cold or fully hot.

Thanks

Let me throw a wrinkle at you. We (most of us) have on-board AFR meters. Those work off an oxygen sensor that responds by producing varying mini-volts relative to the %O2 present in the exhaust stream. So, if we have a plug not (or weakly) firing with all other things considered equal (i.e., fuel delivery) then there would be incomplete combustion....lots of un-burnt fuel and lots of un-consumed O2. Thus, the O2 sensor would read lean because of the higher than normal oxygen level in the exhaust stream.

A plugged injector would read the same (lean) because there wasn't enough fuel to consume all the O2. A unplugged poorly atomizing injector would squirt more fuel than is needed, consuming all the oxygen and reading rich on the AFR.

Run that around your brain a bit. These sensors are only looking at one aspect of the air-to-fuel ratio....that being the O2 content. They are not reporting the ratio per-se.

DISCLAIMER: Logic tells me this is so. Someone tell me differently, as I've wrapped my brain around this more than once.

When this happens next time remove the spark plug caps one at time (run on 5 cyl) and see if it makes difference. Start with #4. If no difference while cap off it would IMO imply ignition problem on that cyl.

No spark would show as lean and colder header IF the mixture does not autoignite in the header which could make the header hotter and not much difference in the AFR. How does that sound : )

Thought I'd close the loop on this finally. I changed plugs, cap and rotor, went through a can of ether looking for vacuum leaks, flow tested injectors, swapped the 4 and 5 injectors, kept the cis gauge on for a few weeks, lowered control pressures, fattened the idle, adjusted the valves, checked head studs, leak down test...sheesh, did I forget anything?

Since none of this helped, I put the control pressures and idle mixture back and gave up. Two weeks ago I replaced my tired looking silicone up pipe and the issue has not resurfaced since. For some reason I didn't catch the leak with the ether. I still have the lower temps on #4 header, but my leak down numbers were good, pulling the plug wire causes rough idle and the plug I replaced looked consistent with the others, so I think the lower temp can be attributed to more air flow hitting that pipe.

Taking her to Blackhawk Farms Raceway for my first DE this weekend, can't wait!!

I took an extra long ride home and saw the same behavior for the first 20 minutes but by the time I got home I was back to normal at idlehttp://www.yono.info/h.jpghttp://www.yono.info/d.jpg