4' vacuum drop M117 engine - PeachParts Mercedes-Benz Forum

Gurunutkins · #1 09-28-2010, 12:22 AM

Hi folks a quick question for your consideration

My car wont go above 2500rpm without missing badly, the idle is perfect with no misses. my car is megasquirted so i can check on fuel and injectors and all systems are going as they should be but the car is running very rich as the vacuum is so low (still runs on map). I have spark when I test but cannot test at higher revs and the pertronix seems to be working fine. the only measurable change i have is that I have suddenly lost 4" of vacuum from 17 inches to 13 inches at idle. I cannot find any reason for the drop - all of the hoses and connectors check out, the brake servo holds vacuum and i have tested with all of the vacuum hoses plugged. The timing is spot on and the chain is new (10k miles). The valves all seem to be fine - the vacuum is steady at 13 inches at idle. I ran my borescope down the inner and outer plenum to check the 8 donuts and they are all intact.
anyone have any ideas?
thanks
Barri

t walgamuth · #2 09-28-2010, 06:06 AM

I'd check valve and ignition timing again.

Mike D · #3 09-28-2010, 07:54 AM

Barri,
When you are running the Megatune program, does the MAP reading change with differing RPM speeds? If not, then double check the vacuum line connection to the ECU.

What was the last thing you did to the car no matter how trivial? Where are you taking the vacuum reading?

You should probably move this thread over to the "performance" or "technical" forums.

Gurunutkins · #4 09-28-2010, 09:00 AM

I'll pull the valve covers again and check but the marks were really close - maybe 2 degrees on the crank. I will also do a compression test to check the valves but she idles fine and the vacuum needle is rock solid, no wobble to indicate a burnt valve

yes the MAP vacuum changes, injector firing changes with MAP change. I considered re mapping the VE table for the new base vacuum and see what happens but didnt as I need to sort out why the vacuum changed. also the TPS is reading fine and the temp sensors for air and water are working fine.

I had just been doing 60 - 70 along a flat piece of highway - slowed to around 30 with a traffic constriction at the bottom of a long steep climb (Peasley canyon rd for those near seattle) the traffic cleared and I accelerated - the car surged ahead and then just dies slowly leaving me in the hammer lane doing 30. I got over to the hard shoulder (of course no one shouted - hooted or gave me the finger

) and pulled off at the next off ramp. couldnt spot anything wrong but my AF gauge was showing I was running really rich. drove home slowly and so long as i dont floor the car its fine. The only thing I can see wrong is the vacuum has dropped from 17 to 13 at idle.
cheers
Barri

230/8 · #5 09-29-2010, 01:18 PM

Just a thought from the old days of troubleshooting...does the vacuum drop further as you raise engine speed from idle up to that point where the engine just goes off the rails? If so, then I'd take a hard look at the exhaust system to see if something internal has failed and is plugging the exhaust.

Way back in high school my crusty old auto shop teacher told us: If the vacuum drops as the engine speed rises there are three reasons...plugged exhaust, plugged exhaust and plugged exhaust. Over the years I learned that he was right...at least about this one particular thing.

Gurunutkins · #6 09-29-2010, 09:17 PM

thanks 230/8 I will check the vacuum as i rev it, I can follow it with the megasquirt MAP display and the vacuum seems to be behaving normally just low throughout the range.

another weird pointer is (mine is a euro spec engine) that the vacuum at the throttle body taken from the vacuum advance feed is 15 inches while the MAP feed at the rear of the intake plenum (the original MAP sensor feed point) is only 12 inches. is it possible to have that much discrepancy across the plenum?
cheers
barri

t walgamuth · #7 09-29-2010, 11:02 PM

I think yes. If checking at a port at the throttle plate I think the vac will be higher than in the body of the intake....venturi effect I believe.

Gurunutkins · #8 09-30-2010, 12:05 AM

thanks Tom - guess so - just wasnt expecting a 3 inch drop.

I will pull her into the shop this weekend and run compression and leakdown tests and double check the timing and exhaust system. Then its rip into the intake plenum time

- I just cannot see anything on the surface that causing it.

thanks for all of the thoughts
cheers
barri

Tomguy · #9 09-30-2010, 03:19 AM

Venturi vac @ the TB is completely unrelated to the vac in the manifold. They will vary wildly.

A drop in manifold vac like that either indicates a leak or another issue (like excessive backpressure as suggested, valve timing off or valve clearances off, etc). Ignition timing still spot-on? Sudden failure of the aux air valve? If it were a big vac LEAK I'd expect a raise in engine idle speed. Is it faster? 4" is enough to notice faster idle unless it's caused by something else...

Gurunutkins · #10 09-30-2010, 09:23 AM

Thanks Tom - I thought with the euro takeoff just below the throttle plate there might not be so much variation.

no change in idle but that is probably just the megasquirt compensating. with the TPS reading idle it will try and hold the revs lower. This is the car that i modified to run with an electric AAV run from the dash so I can close the AAV down to the warm position so i know its not an AAV leak. I run fuel pressure and AFR guages on the dash so I can see the car is running really rich when it is running at idleand my fuel pressure just sits at 32lbs irrespective of revs. The ignition timing is exact for idle but I cannot check at 3000rpm as it is very wobbly above 2300 rpm and unpredictable even under no load. It will get right up to 5.5k with no problem but bogs down at 2.5 and then just screams up above 3k. Pulled all of the injectors on their rails and checked for injection and all are working fine, pulsing right.
cheers
Barri

Graham · #11 09-30-2010, 10:29 AM

Quote:

Originally Posted by Gurunutkins

Thanks to - I thought with the euro takeoff just below the throttle plate there might not be so much variation.

no change in idle but that is probably just the megasquirt compensating. with the TPS reading idle it will try and hold the revs lower. This is the car that i modified to run with an electric AAV run from the dash so I can close the AAV down to the warm position so i know its not an AAV leak. I run fuel pressure and AFR guages on the dash so I can see the car is running really rich when it is running at idleand my fuel pressure just sits at 32lbs irrespective of revs. The ignition timing is exact for idle but I cannot check at 3000rpm as it is very wobbly above 2300 rpm and unpredictable even under no load. It will get right up to 5.5k with no problem but bogs down at 2.5 and then just screams up above 3k. Pulled all of the injectors on their rails and checked for injection and all are working fine, pulsing right.
cheers
Barri

barri,
when you rev the engine, can you see the centrifugal advance operate? Even at 2500rpm, you should see 25 or so BTDC.

I don't think that measuring vacuum at throttle body is meaningful - the airflow and turbulence affects the reading. The best place is via tee into the MPS line. ? I sometimes connect my Mityvac to the connection to the climate controls at back of engine - reads slightly lower vacuum there. Is there any chance your new pressure sensor is leaking

On my car with timing set to about 28deg BTDC at 3000rpm, I am at about 16" Hg. I can get it to about 18" if I fully advance (hard up against end of slot). I used this site back when I was trying to diagnose a vacuum problem:
http://www.secondchancegarage.com/public/186.cfm

Is engine running on all cylinders? All injectors & plugs firing when they should? I once had rich mixture and it was caused by partial failure of the trigger points (you don't have these anymore?). Car became very sluggish.

Maybe a compression test might tell you something?

I am sure YOU will figure out what is wrong!

GaryF · #12 09-30-2010, 02:10 PM

Quote:

Originally Posted by Gurunutkins

no change in idle but that is probably just the megasquirt compensating. with the TPS reading idle it will try and hold the revs lower.

Confused, how does megasquirt manage the revs. Surely it can only manage the mixture at an rpm determined by factors beyond its control?

Graham · #13 09-30-2010, 06:01 PM

I was in Princess Auto (like HF) today and they had mechanical vacuum gauges on sale for C$15.99. Guess what my next project will be! But something has to go - probably the clock, because ashtray area already has fuel pressure and A/F ratio!

Figure vacuum is probably the best indicator of engine condition/leaks.

Gurunutkins · #14 10-01-2010, 09:20 AM

Gary - you are right Megasquirt cannot control the revs but once the car is out of the warm up cycle dependent on where you set the ego kick in (mine is 900rpm) and how much control you give the MS to control mix (mine is 20%) the MS will control the injector on off timing to comply with your VE table and where the TPS position. So even though I am still running rich my revs could be running some factor of 15% below where they might be as the MS compensates. I also have an electrically operated AAV on this car and have it locked all of the way down at the moment. One of my other other 450's (this one is the 350 euro M116) is a US spec M117 and I have the MS1 but with the fast idle board so it controls and electric AAV for me and that will control revs particularly during the cold start phase to hold the engine at a specified warm up rpm. Maybe this will be the kick I need to complete the MS2 board i have and get the EDIS installed

although I still think this is a mechanical problem

Graham - haven't talked to you for ages - busy at work. I will check the dist and maybe pull it and do a visual check as well when i am checking the valves and chain again - could be the reason as i am seeing a lot of wobble in the timing above 2000 rpm. my car sounds like yours

I have an afr and pressure gauge on the dash where the ashtray was and a vacuum gauge in a pod next to the console. The pressure gauge was one of the first - I like to make sure I haven't burst a pipe and am not spraying petrol all over the hot engine whilst sitting in the car wondering what the smell is and why the car isnt performing properly

I'll get her into the shop tonight and start playing
cheers
barri

PanzerSD · #15 10-01-2010, 11:37 PM

Quote:

Originally Posted by 230/8

Just a thought from the old days of troubleshooting...does the vacuum drop further as you raise engine speed from idle up to that point where the engine just goes off the rails? If so, then I'd take a hard look at the exhaust system to see if something internal has failed and is plugging the exhaust.

Way back in high school my crusty old auto shop teacher told us: If the vacuum drops as the engine speed rises there are three reasons...plugged exhaust, plugged exhaust and plugged exhaust. Over the years I learned that he was right...at least about this one particular thing.

Isn't the vacuum supposed to drop as engine rpm increase? I mean you are opening the throttle valve releasing that negative pressure.. the Trans modulator also requires an Hg drop to properly shift the car. It's the trans's way of sensing engine load. Correct me if I'm wrong.....please

I'm having some issues with misses and such, and Vac is one of the suspects.
I'll be glued to this thread