Lean at low revs/rich at high revs - PeachParts Mercedes-Benz Forum

pentoman · #1 07-18-2008, 01:36 PM

Hello all,

I've a 190E with the 2.5-16 engine. I've been tracing a very mild but noticeable poor throttle response and flatness low down for a good time now.

Today I unplugged the coolant temperature sensor and plugged in a resistor. This resistor is the correct value to fool the ECU into thinking the engine is at 15 degrees C i.e. stone cold. The interesting thing is how it drives when I do this. The response and power down low is better (under 3000 rpm), very noticeably. Unfortunately over 3000 rpm it's way off and completely lacking in acceleration, it bucks and jumps when you back off the throttle too.

I think that the resistor trick is richening the mixture up? Therefore my engine must naturally be lean at low revs and/or rich at high revs, right? The mixture is adjusted to be correct at idle when I'm not messing with my trick resistor but obviously this doesn't mean it's right when you're on the throttle...

Opinions?

mbdoc · #2 07-18-2008, 02:29 PM

Have you tried adjusting the EHA? Even 1/6 turn clockwise will make a big difference,

pentoman · #3 07-19-2008, 04:39 AM

I have tried it but only by a certain amount of guesswork to see what happens. It did make a difference but at the same time something else would get worse.

This is actually my third EHA and 2nd complete fuel assembly, (not brand new, taken off 100k mile cars), and this problem remains present, so I guess I want to know what to look at. I will check all sensors/switches this weekend but apart from that am totally stumped.

MDE3 · #4 07-19-2008, 11:23 AM

Could be your O2 or even your ecu (or both)....sounds like how the car will run if you disconnect your eha and tune it manually...

Not sure if it will work for your 2.5, but to test my 2.3-16V we installed an ecu from a 2.6 (could not find a good used ecu for a 16V)...guess what..it ran fine...and as I had such a hard time finding a good used ecu for the 16V, I just left it in...been running with that ecu for more than 2 years now.

Some have claimed to be able to resolder the connections on an ecu, but when we took my faulty one apart, we could find no visible cracks....(under magnification)..that's when we decided to just try an ecu from a 2.6 to see what happened.

Before the ecu change, the poorly running motor managed to clog up the O2's very quickly...so the car would run like a raped ape for 10 minutes to 2 hours after replacing or repairing the O2's..then back to the same status. I replaced the O2 three times before we figured out that the ecu was causing the faults, and making the exchaust so rich it would blind the O2's....I was able to salvage the second O2, bench test it and clean it, so I didn't have to buy a third one.

Once we got the ecu done, we tuned it normally, and everything has been running fine since (until timing chain guide just failed).

pentoman · #5 07-19-2008, 03:14 PM

Mmmmm... interesting. I always thought the ECU would be fine. If you unplug EHA connection it runs crummy therefore I'm certain the ECU does something, but perhaps it is dead or ECU related. Since i've reconditioned the head and replaced fuel system except accumulator and fuel pump, it drives with this same issue still so perhaps you're right. Or could be intake manifold gaskets. Or air plate potentiometer but I replaced that too with the one on the spare unit I bought.

Hmm.

n

jhodg5ck · #6 07-19-2008, 08:43 PM

I had the Similar problem on an M103...checked sensors/swapped modules/EHA etc, etc.. Turned out to be an O2 sensor.. One Mustang sensor later everything was A Ok...A/F ratio's right back where they should be.

For a reference, we were getting 14.3-14.8 @ WOT and 12.9-13.1 @ steady state cruising, high 14's low 15's @ idle.

I'll be curious to hear how you make out.

Jonathan

pentoman · #7 07-20-2008, 04:31 AM

I don't have an O2 sensor so it can't be that

jhodg5ck · #8 07-20-2008, 09:59 AM

Really?!? your car is just straight CIS???
I thought all 16V's were CIS E...?

Jonathan

pentoman · #9 07-20-2008, 05:20 PM

I've not understood this for ages but am now pretty sure CIS-E (KE Jetronic) is jetronic using an EHA type valve for adjustment - which is electronic. Plain CIS (K Jetronic) uses a warm up regulator like the old 450SLs amongst others.

With that cleared up, I have KE (precisely, upgraded to KE III for the 2.5-16 according to the brochure whatever that means) but it's without O2 sensor. This is completely normal for almost all 16vs in the UK and europe. In UK there was no requirement for catalytic converter until 1993 so the 2.5 doesn't have one and they decided not to put a O2 sensor on it either. These days you wonder why however I'm sure it made sense then.

jhodg5ck · #10 07-20-2008, 05:37 PM

I'm just surprised..the O2 makes for much more accurate tuning/adaptation of the fuel system..not only for emissions but for power as well!

All the CIS E Euro cars I've come across here came equipped w/ O2's from the factory.

That aside, have you checked your air flow plate sensor and TPS to see if they're working properly?

Jonathan

pentoman · #11 07-21-2008, 05:32 PM

Well the whole fuel assembly was swapped for a used one, including the air plate unit and the throttle position sensor (the potentiometer one) that goes on it. I do plan to test it but it's less likely that two of these are faulty and giving me the same fault don't you think?

To me having grown up with 02 sensor cars this is odd not having one, but then again carburettor cars do/did a great job for power, performance and driveability so I don't see that this should suffer for it.

psfred · #12 07-23-2008, 12:01 PM

Find a repair your vacuum leak.

You will then need to set your idle mixture correctly using a CO meter.

Peter

pentoman · #13 07-24-2008, 05:57 AM

I have a vacuum leak?? How do you know?

Is this the same as an air leak? Would an emissions meter show up the excess O2 in the mixture at idle if I had an air leak?

pentoman · #14 08-05-2008, 11:23 AM

Quote:

Originally Posted by jhodg5ck

That aside, have you checked your air flow plate sensor and TPS to see if they're working properly?

Jonathan

OK I tested those - TPS sits at about 0.72V when engine idles warm. The voltage rises to 4.8V smoothly if you push the flap down. So I think it's Ok.

I also went over a lot of the sensors, with the wiring diagram, and tested them at the connector pins of the ECU plug. Air temp sensor, coolant temp sensors, throttle switches (both), plus some other plugs at the fuel pump relay/OVP. It all was fine.

Maybe I've got an air leak between the intake plenum and the throttle body?

To be honest it's not too bad now, seemed to improve after I tested those components oddly. I am extra picky though so keep investigating and it's good to learn too!