SQLGuy

Hi folks,

I've been fighting with a problem on my '88 S4 5-speed, which is supercharged with a Murf 1.5 kit. Basically, this issue is that, when I let off the throttle (like when coming to a stop at a light), the engine speed drops a lot, and, if the A/C is on, it will likely stall.

A bit of history: Before supercharging, idle control was fine, and the car would catch idle just fine. However, there was a very intermittent problem where, like maybe six times in a year, the car would catch, then die, and flood, when starting cold. Once flooded, it could be started easily by removing the fuel pump fuse, but otherwise would be very difficult to get started. Besides that very intermittent issue, the car would normally start just fine with a turn of the key, and ran well.

With the current situation, one additional symptom noted is that, with the engine warm, if I rev to 2500 RPM or so for a few seconds, I'll see the AFR close to Stoich (14.7 - 15). If I then let off the throttle, the AFR will peg to 21.9 (full lean) until engine speed drops to 500 or so, then will spike to about 10.0 (maybe even a bit richer). If the A/C is off, RPMs will generally keep dropping, but then catch, climb back, and, after a bit of oscillation, stabilize at 675. It takes about 10 seconds for AFR to climb back to 14.7 and stabilize.

Here's what I've tried so far in troubleshooting this issue:

1. Pressurized the manifold with shop air at a few PSI. I found two small leaks (one at a the line running to the front damper, the other at the driver's side Y-connector coming out of the MAF boot). Fixed both, but it made no difference.

2. Swapped in the MAF from the GTS (and swapped over the S/C MAF mod torus). No difference.

3. Checked the coding plug on the LH. It's correct, as is the wiring in the plug running to it.

So, at this point, what I'm seeing is that the LH seems to be overfueling in open loop, at least when returning to idle, and is responding to Lamdba input to correct mixture back to Stoich.

Tim Murphy tells me that the map is correct. If the map is correct, and the MAF is metering air correctly (which it seems like it should be), and the open loop mixture is this rich, I'm thinking that the fuel pressure is too high. The injectors are the 42lb/hr units that came with the kit. One thing I noticed is that the injection system is much louder than on the GTS... sounds kind of like a dry lifter ticking. You feel the vibration form the injection pulses if you touch the damper or the fuel rail. I'm not sure whether this is normal when larger injectors are installed.

Anyway, vacuum lines are connected and correctly routed to the FPR and dampers, and I have checked the lines for fuel - they are dry. I am wondering, though, whether my FPR might be bad or sticking and causing higher than expected fuel pressure when the throttle is closed and the manifold goes to vacuum. Has anyone seen such a failure mode? Any suggestions on futher isolating the cause of this issue?

Thanks,
Paul

John Speake

Maybe a slight misfire, which would pump some unburnt fuel into the exhaust and make the WBO2 read too rich ?

What type of WBO2 are you using ? Is the NB signal to the LH from a seperate NB sensor, or derived from the WB unit ?

SQLGuy

Hi John,

It's an Innovate, and it provides both the gauge display and NB signal to the LH.

Its reading has been confirmed by my mechanic's emissions "sniffer".

What would cause a consistent misfire under these circumstances?

By the way, since the system is open loop during warmup, I can also see there that, from cold start it's running about 9.5:1, and is still a bit below 12:1 when it gets warm enough to go closed-loop and regulates to Stoich. Is this typical enrichment for warmup?

SQLGuy

I spoke with my mechanic about this as well. In his experience, the FPR and dampers either work or fail catastrophically (cutting off all fuel flow). That makes my symptoms being a fuel pressure problem less likely, I guess, but I still think I need to get a gauge on there and see what the pressure is doing under various circumstances.

John Speake

I don't have any real experience as to warm up enrichment with cat equipped cars as they are quite a small % of 928s sold in the UK.

12:1 does seem richer than one might expect though..... a gauge sounds like a good idea.

Lizard928

I have a situation with my car since putting in 60# injectors that it will start perfectly, but if I dont give it just a tick of gas immediately after its started it will stall and be too rich to start until I hold my foot to the floor. The problem that I find is that the after start settings are not for enough cycles. In other words the afterstart should hold longer and this would go away. If it were possible to double the length of cycles that the ISV stays open for at start I would not have this problem.

As to your problem of 2500-idle, it sounds like your fuel cutoff system has a fault. The fuel cut off should not cut fuel below 2500RPM.
The reason that it is going to 10:1 is because of the base fuel map (not the adjusted map), it has a very big spike in the bottom corner of the map which cannot be pulled out with the sharktuner.

I have an issue with my car that is similar, if I am at high RPMS with load and then chop the throttle, and push in the clutch the engine will stall. I believe it has to do with fuel cut not recovering fast enough as well. But just modify my driving slightly and never see it anymore.

T_MaX

Trashy signal or low output from alternator?

SQLGuy

Hi Colin,

Can you elaborate a bit on the fuel cutoff? Are you saying that closing the throttle at high RPMs should only go into fuel cut until the engine is at 2500, and then will begin feeding fuel again? If so, will this further decell be light fueling, that might be out of range for my WB, or should it go back to Stoich?

I'm definitely not seeing any measurable fueling until engine speed falls below target idle.

Thanks,
Paul

76FJ55

Could you have your throttle body bypass screw to far in? this can cause a excess drop in rpm before the IAC valve has time to compensate. My 85 used to drop and then climb back up to stable idle, till I adjusted the bypass screw. Try backing the bypass screw out a couple turns and see what happens.

Bill Ball

There is no bypass screw on the S4 throttle body as there is on earlier throttles, like 85-6.

I believe the no-throttle fuel cutoff is above 1400 RPM for LH, not 2500. Regardless, you can disable that with Sharktuner.

Whenever I've had a misfire, I've been able to hear it in the exhaust note.

It sounds like there was an intermittent problem with the LH before you supercharged. Have you or can you try an alternative LH box (with your "tuned" chip).

Lizard928

Yup sorry, I meant to type 1500 rpm not 2500, and with a sharktuner you can disable that option.

soupcan

You don't have both on one right?
There are two signal wires, one for the wideband and one simulated 0-1V for the ecu.

SQLGuy

No, I don't have both on one. The analog output currently set for NB simulation is feeding the LH, and the one set for WB is feeding the gauge.

Update, though: I just checked the fuel pressure with the engine off and am reading 59 PSI, which, unless I'm misreading the manual, is about 20 PSI over the max expected reading. Bad FPR?

Ugh. Wrong section of the manual. Looks like my pressure is still maybe just over the high end of the range... but not by much. S4 and later should be 3.8 bar +/- .2. 4.0 Bar is about 58 PSI.

Edit: I just tried a MityVac on the FPR to see how it responded. It looked correct for the amount of vacuum I was able to apply. 18 in Hg of vacuum reduced the pressure from about 58 PSI to about 49 PSI. The pressure seemed to respond immediately to the applied vacuum. Next step is to see what's happening as far as vacuum at the FPR and pressure on the rail when the engine is running and the problem conditions are simulated.