190E lazy throttle reponse
 

I have a 16v, on which over the last month I have reconditioned the top end.
See http://www.benzworld.org/forums/w201-190e-d-class/1321311-head-coming-off-tuning.html
for all the pics. The compression had been low. I had 4 new valves and reseated all valves.It's all back together now and better than ever. The torque low down is much improved, 3500rpm flat spot gone, and throttle response much better.

However, the throttle response is still rather lazy. It takes between 0.5s - 1 second from hitting WOT to the engine giving full acceleration. It just isn't that response to the pedal. This happens at high and low revs, but worse at low revs. Any idea what can cause this? The mixture is spot-on. I tried unplugging the EHA... it was not very different, perhaps a little bit worse. The other thing I've checked is that the air flow meter plate is sitting in the right position and is not sticking. I don't know what else causes poor throttle response? The performance is fine when it arrives!

Disconnect the battery for about 10 mins first to see if it is not just a computer reset that is needed.

Can you check intake vaccum ? Should be 16-22 inch Hg at idle. If the vaccum is low, could be a clog in the exhaust, most likely the catalytic converter.

Also you can measure the current at the EHA. If you just have the ignition switch in run position, there should be between 10-20 ma thru the EHA. At idle the current will be lower. Now if you blip the accelerator, you should get greater than 15 ma thru the EHA. Note that the current must be measured in a series circuit not in a parallel circuit like measuring voltage. This test should be performed when the car is cold.

Does your car have a EGR valve ? Check the valve and the vaccum that operates the valve. At idle, the vaccum that controls this valve should be less than 2 inch Hg, i.e. the valve must be closed. At about 2000 rpm, the vaccum should be between 4 and 6 inch Hg, enough to open the valve. Malfunction in this valve's operation will cause problems at idle as well as rough acceleration. Make sure the car is cold when testing this.

Check the air sensor's potentiometer. The car is off. The resistance between the center and one of the neighbouring pin should be about 1 to 2K when the plate is all the way to the top. Now if you gradually press the plate, the resistance should gradually increase, there should be no jumps greater than 2K. If it is smooth, check the voltage across the same two terminals. Now the car is on (and you have to also connect the sensor's cable back to the sensor, can use some alligator clips). At idle, the voltage is between 0.5 and 1V. Now as you gradually depress the gas pedal, the voltage should smoothly increase, there should be no erratic voltage change.

Check the coolant temp sensor also. This sensor sends engine temp info to the fuel computer (as well as the ignition computer). When the engine temp is cold (about 20 degc), the resistance should be between 2.5 and 3.5K. When the engine temp reaches 80 degC, the resistance should drop to about 250-350 ohms, a factor of 8 to 10. Check the sensor's cable, if the cables are broken, the car will go into a limp mode.

I am not familiar with your car but am assuming you have the same system as my 87, 260E.

It's broadly the same system yes. I think on a 16v there is more to the ignition computer - it's got an intake air temperature sensor and stuff. But pretty similar.

Thanks for the help I'll look into vacuum problems The previous owner had all vacuum lines replaced but there could still be a problem.

How does the vacuum affect the running of the engine, what does it do?

Russell

In a simple view, vaccum is produced in the intake manifold if the engine is operating correctly. Thus vaccum at the intake manifold can be a good tool to diagnose engine problems including leaky valves, timing problems and exhaust problems. When the exhaust system is not working properly, then the vaccum level (I will only refer to the intake vaccum) at idle drops dramatically, for e.g. if there is a clog, the vaccum would be much lower than 16 inch Hg. And normally, the vaccum drops some amount as you acclerate, because the throttle opening lets the air into the intake. But if the exhaust is clogged (or if EGR is always open, it should be closed at idle), it would dive down to zero.

The vaccum created at the manifold is also used to control other vaccum driven elements in the car such as your a/c vent control flaps, automatic door locks, brake booster etc.

On a diesel engine, there is normally a seperate vaccum pump because a diesel engine does not produce enough vaccum for anything useful.

And vaccum is one of the easiest way to control mechanical elements that are widely distributed across the car.

In short, vaccum levels at the intake manifold are critical for diagnosis as well as function of the car. If you dont have a mytivac, it is worthwhile to have and it has a little manual with it which lists all the engine functions you can diagnose with it, pretty handy tool.

OK thanks.

So is the amount of vacuum used as an indication of the load on the engine? Why is it there on these cars, when there's also in air flow meter which would measure the load on the engine? Are the two combined?

There is for example a vacuum pipe to the ignition control module (I believe your car doesn't have this). Is this so the ignition module knows the load on the engine and can adjust ignition timing accordingly?

There's also vacuum lines from the sensor block (top front of engine), the bottom of the intake, the throttle body. Why so many? What's it all for?

Of items previously mentioned, I would pay particular attention to the AFM potentiometer. Enrichment during rapid change in airflow demand is its primary function. If you are not getting any of the other symptoms of wear or malfunction - erratic idle, etc - then it may just need adjustment (rotation). Monitor its function by monitoring the center pin voltage - it should meet calibration value at idle, and change smoothly but rapidly under a throttle stomp.

Steve

No, vaccum is not used as an indicator of load (I am not sure what you exactly mean by load) but rather used for control of various mechanical valves. And a useful indicator of the functions within the engine, because only if these functions are normal, you get certain values of vaccum.

The air flow sensor just tells the fuel injection computer how much air is flowing to the engine, and the fuel injection computer can then adjust the air/fuel mixture accordingly. More air will flow as you press the gas pedal. The throttle valve when opened, will pull the air sensor plate down and more air will flow thru the intake. The pulling of the sir sensor plate very much depends on vaccum in the intake. If the vaccum is low, you will not have a good throttle response, lots of hesitation.

There are a lot of electrically controlled vaccum valves, they let vaccum pass thru them when activated by some voltage. These valves can be used to control vaccum flow electrically and hence control various vaccum controlled valves. The ignition control unit uses vaccum for correcting the timing of electrical pulses going to the coil. Vaccum, I think is also used for some control of transmission functions.

Vaccum is a key element of even the most modern car.

OK thanks for that. "Load" is the load on the engine, and as you described, when there's a load on the engine there's vacuum in the intake which pulls the flap down, gotcha. But I was wondering if this amount of vacuum generated is used as a signal (via the vacuum pipes) to anything, apart from the ignition module as you already identified.
I have tested my AFM potentiometer and the resistance values seem fine... Also unplugging it makes the throttle response even worse. So it's certainly working, but maybe some trial adjustments would reveal something? Would that be worth trying or a bit risky?

Because of its internal design (built-in series resistor), resistance measurements of the AFM pot are not that revealing - in-use voltage measurements are needed for evaluation. Still, your other tests seem to indicate you shouldn't concentrate too much effort on it. Rotating it experimentally should be proceeded by very carefully scribing an index mark, so you can accurately return to 'go'.

Maybe time to scope the engine - e.g. you may have lost ignition advance or valve timing off.

Steve

I am not sure why the resistance test would not reveal an open spot. This is actually a test that MB manual recommends and is easy to do with the connector removed and car off. Dont make any adjustment on the pot if the resistance is between 1K and 2K when the plate is all the way to the top, just check if the resistance changes smoothly as you press the air sensor plate.

You can check the voltage but because removing it has an adverse effect, one could assume that it is less likely to be a problem. The voltage on the center pin when the car is at idle should be between 0.5 and 1V, ideally 0.7V. Then the voltage should smoothly increase as you depress the pedal. No erratic change in voltage. The problem is when you do find an erratic change in voltage, the only way to tell if it is the pot or other throttle related problem is to do the above resistance test. If you dont get any voltage, then there is a problem somewhere else.

Check the EHA operation and coolant temp sensor next. They both are critical for acceleration enrichment.

Right thanks again Saumil.

I just briefly looked at EHA current with engine warm. It was 0 ma and when revving the engine stayed at 0ma.

The meter is definitely working and was hooked up correctly.

I find this a bit odd since the EHA definitely does something - with EHA unplugged when cold the response is very poor and it can misfure. If I connect the EHA the wrong way round things are even worse (presumably because it's leaning when it should be richening and vice versa!)

I will investigate further this afternoon.... and maybe check the wiring/continuity.

Russsell

OK Ignore the above post.

I tested it again. Engnies Temp was about 40 degrees. With key on engine off current is +59ma . The service manual addition for the (2.3) 16v engine states 50ma at 20 degrees. So it's not miles off but a little perhaps.

At idle it's exactly 0ma.

Opening the throttle makes it jump to +40ma or so.

As the revs come down it goes to -50ma (Deceleration shutoff)

So that seems to be functioning about right or not miles off. Resistance was 20.5 ohms, a little over spec of 19.5 ohms but not a problem?

I did notice, when I accidentally left the engine temperature sensor disconnected and went for a drive, that the response problem was magnified. I suppose this is expected and doesn't tell me much?

It's annoying me a lot :behead:

EHA is not a problem in your car. Have you done the voltage check on the air sensor potentiometer ? Check the intake vaccum also.

Here in Gloucestershire, England there are huge floods and half the place including 300,000 homes has no water supply, so work is closed..

So I went to my parents' garage and messed with the car for a day. I found a 6kOhm resistor and hooked it up to the coolant temperature sensor (injection side). This fools the engine into thinking coolant temp was 5-10 degrees C. Then I went for a drive. The poor response was gone - much improved, Presumably because the car was running a lot richer. I switched back to the proper temp sensor, restarted, and instantly the poor response was back. I've tested the coolant temp sensor by the way and it checks out.

So there is I think a lack of fuel richness for instantaneous throttle response. I then swapped the EHA with the one from my other 190E (which has a perfect throttle response). This made no particular difference so I agree, it doesn't seem to be the EHA. That said this 190 is an '86 8 valve and therefore has a slightly different EHA part number..

So assuming the EHA is ok, should I assume that it is not being instructed to richen enough (problem with throttle potentiometer, air temp sensor, etc etc or maybe the ECU as an unlikely last resort)? Or could it be something to do with the mechanical side of things - fuel pressure and so on? These are harder to check...

Saumil - I will try to test potentiometer voltage as you suggest. For the record the Bosch specs for 2.5-16 I have say 0.2 - 0.5V and 5V, for pins 1-2 and pins 2(?)-3, respectively.

Isnt the temp sensor test you did giving you the clues. It is likely to be just a fuel mixture adjustment that is off. If it were the air sensor pot, it would not have worked well when you attached the resistor. And if the coolant temp sensor resistance changes properly with engine temp, and is close to 2.5-3.5K at 20 deg-C, then just enrichen the mixture a bit and most likely you would be fine.

But the fuel mixture is set correctly - in fact ever so slightly rich.
Richening the car up would only get around the problem, not solve it.

This afternoon I tested the air flow sensor potentiometer. It checked out - 0.4V at the specified (near rest) position, nice and smooth up to 5V throughout motion. Also removed ECU and checked continuity for wires to EHA, WOT/idle switch, and air intake temp sensor. I'll check the coolant temp sensor when cold but think it's correct.

So I am 90% certain everything electronic is working correctly.

0.4V is that what you get at idle ? That may be a little low. Can you adjust it to be 0.7V ? Did you remove the sensor at any time ? If you did then, you can adjust the rotation of the sensor a little by loosening the mounting screws to give you about 0.7V at idle.

0.2V to 0.5V is the spec for the 16v engine, according to the US Mercedes-Benz 102.983 service manual.

As it happens on this machine here I've got Bosch's all-informative ESI system which lists this for the 2.5-16 potentiometer adjustment (see attachment). I checked and it shows the same data for the '87 260E.

Bosch's information a bit more circumspect- the base position depends on the supply voltage. My supply voltage was 4.7V so my measured voltage should be approx 0.5-0.8V.

However, it's not quite that simple. These instructions do NOT state anything about moving the sensor plate down to a certain position to perform the measurement. Whereas the Mercedes service manual says to line up the top of the metal plate with the bottom of the shiny cylindrical part of the intake hole and take the measurement from there.

So now I'm confused.....

For information, this is what Bosch ESI says for the position of the air flow sensor (see attachment). No idea how to interpret it though!!

I haven't removed the sensor. I would like to try adjusting the potentiometer perhaps, so long as it won't mess everything up! Is that how you adjust it - just loosen the screw? I can't even see how to get to it, on a 16v it's hard.

O.K., so the range is different for your car. No dont remove the sensor, it seems to be working fine.

I just saw the specs you put, yes the range does depend slightly on the incoming voltage, so 0.4V is on the low side even borderline off, it would not hurt to atleast make it 0.65, but do it as they say, using the trim pot screw on it.

The trim pot screw may be hidden behind some black material, they dont want it to be easily exposed and hence moved. The adjustment by slightly loosening the mounting screw is a very coarse method, only do it if the trim pot does not work for some reason.

The pot seems fine, and close enough in setting I wouldn't bother trying to adjust. The problem is elsewhere.

Steve

I agree, the dynamics of the pot are o.k., however the baseline is 40% off, which in my opinion is substantial. And I agree there is likely some other problem also. Even if the temp sensor comes out o.k., check that its signal is reaching the ECU and ICU, check continuity.

Yep I've tested continuity to coolant temp sensor :/

Today we're testing the ignition timing. My Dad reckons it is off when he had a quick look with a timing light yesterday. However we're being hampered in testing this because the area with the TDC markings on is damaged - half the readings are missing :o. The stud (for some sensor I can't remember which) is there but the TDC marking missing, it looks like it's shattered away!

How could the ignition timing be off, and what could cause that which I could investigate in the meantime?

Did you replace your fuel filter?

Ignition timing was on spec.

No I haven't replaced fuel filter. My Dad reckoned the fuel filter would be unlikely to cause this sort of problem (makes sense... the problem would most likely be there the whole time, or be worse at higher revs/fuel flow). Maybe I should replace it but I'm averse to randomly throwing money at a car to attempt to fix it.....:grim:

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OK well with everything checked and verified, I took a deep breath and went for tweaking the EHA adjustment. As I understand, this varies the chamber pressure (lower pressure is supposed to be 0.4bar lower?). I wasn't measuring the chamber pressures so it was a bit hit-and-miss.

It's currently set to be richer than initially. This has improved the engine response greatly. However it's still not perfect with a very slight delay and without a crisp throttle response.

Russell

Well I was probably over-eager to judge there (and maybe again now!). Went richer again and problem got worse. Right I thought, I'm sick of this, let's go wild.

So the EHA is now adjusted it hugely CCW (about 3/8 turn) - from what I read this means leaner.
And what do you know, the low down torque is massively improved and I am certain of this. It doesn't have the same throttle lag/delay either but can occasionally mildly hesitate.

Further adjustment required. I can't work out why it's better this way? Perhaps my fuel pressure regulator is actually the problem.