Lilbullet

Ok folks, so I asked this question on the SLK 55 forum but didn’t get any response so I’m you guys can help.

My SLK 55 is MY05 model with 120k miles on the clock. It previously had K&N filter and the plugs were changed a year or so ago.

NO DTC’s and the fuel trims are fine at normal running as is the O2 sensor readings.

While on the dyno, with ambient temps around the 25 degrees C, the car appeared to be running out of steam as shown in the plot below. Unfortunately I was able to get a copy of the datalog but the tuner, who has tuned several including a Cup and Black series, felt that the issue was likely the MAF and K&N. Both have now been replaced but I’m trying to figure out what else I else I can discount before putting it back on the dyno.

I’m guessing a vacuum leak and/or fuel pump/filter, as the car has previously had a replacement crankshaft sensor at 90k miles.

any suggestions and guidance welcome.

Thanks

Acta_Non_Verba

Usually when all else seems good, no CELs, running well in mid-RPM range, but going flat on the top end is fuel starvation. So I would have fuel filter/pump higher on my list of suspects.

You cannot use fuel trims here as when accelerating even at a moderate rate, closed loop opens and power enrichment takes over. PE runs on a fixed fueling schedule which will be richer than stoich. If the fuel pump cannot keep up with the demand on the high end the AFR will go lean at the exact wrong time when you need fuel, but there are no safeguards once out of closed loop. Safeguards such as a P0171 or 174 don't work in PE.

Also, lower on my list might be fuel injectors that might be dirty. I wouldn't agree with a vac leak. It would be a little lower on my list BC a vac leak would likely give you a lumpy idle first, but have less impact on the big end.

EDIT: I thought I'd add this line to this post - before I started just replacing parts like fuel pump I would first get a gauge on the fuel line somewhere past the fuel pressure regulator and see what pressure I have at the high end when accelerating. BTW, could also be the fuel pressure regulator.

Acta_Non_Verba

I did a quick lookup of a car I was tuning. Here is an example of a DL pull I did. It demonstrates leaving closed loop to OL (open loop) and when PE takes over. The data shown is representative of the moment where the vertical line is positioned in the log. You can see I was at +5000 RPM and accelerating The RED ARROW shows the trims status as OL so trims have quit working and PE is in place.



In this particular pull, BC I was accelerating hard, OL happened much sooner than the white vertical, earlier on this log where the fuel trims (bottom two graphs) went flat and AFRs (top graph) went steady richer

Lilbullet

Thanks for this, it’s makes a lot of sense. Looking through the service book, it recommends a fuel filter change at 150k miles or 15 years, so I’m guessing it’s due now at 17 years old. I’ll try that first before attempting the fuel pump replacement.

Out of interest, is there a way of using my scan tool to check for a defective pump/injectors or filter, or once the car is in PE it either has enough fuel for the performance or it doesn’t? Any suggestions here would be appreciated before I start throwing money at it.

I was originally planning on booking a dyno session to see whether replacing the MAF and air filter has made any difference, but I’ll hold fire until I’ve investigated further.

Thanks

Acta_Non_Verba

If it were me, I'd do what I suggested in my EDIT. Get a gauge on the fuel rail somewhere past the pressure regulator and see what you have.

I'm not discounting a MAF. Yes, it could have a flatspot in its upper range. I've done some bigger build with NA engine going to FI and when I tuned them I sometimes would remap the MAF but frankly, if I had a fuel starvation problem in PE I would rather change the injector data. The MAF remap only works if I wasn't getting a smooth AFR, but spikes instead. I could usually iron out those wrinkles with a MAF adjustment.

Here is an example of a stock MAF output. The one below it is the same MAF I reworked BC when I boosted I was getting back drafts on the MAF, bounce back from intake valves slamming shut and it was giving me an irregular MAF output, so I just corrected each place where I needed to ensure a steady AFR in PE.




This is all a "fish story" for your case though. Unless you've done some very radical change to this engine, I would concentrate on fueling components first including the fuel injectors (as in dirty/fouled). If all that checked out I would then maybe move to air flow.

You more need a data log capability. Basic scan tools can't really do much here.

Lilbullet

By way of an update, I took the car for a dyno session this morning. The upshot was that the car managed 327bhp (crank) and the datalogs were fine, although a little rich up top.

The tuner pointed to suspected clogged cats. The first run was 275bhp - then a huge plume of black smoke, the smell of eggs and what appeared to be a restriction cleared. Torque numbers seem fine.

From the looks of things there’s still room for improvement. Perhaps sports cats, headers and cat delete - either way the car seems fine.

Acta, do you have a view based on the attached plot? My thinking is that the last plot showed max power at 4k, 1750 short of where it should be. I guess in the real world one would want peak power earlier in the rev range than later.

Acta_Non_Verba

The theory about clogged cats is a DEFINITE possibility and conditions would look something like the dyno in your first post. I only had it happen to me one time on an older Corvette before diagnostics was possible. Took me forever to diagnose and hasn't happened since, but definitely could. Cats are typically built much better than they used to be. I would b adding that to the list.

This is why it is so difficult to diagnose anything online as so many things are missed when you cannot see and hear the car running.

All the Best to you getting it the way you want

Acta