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Reason for investigation - car bogs down (smoothly, not like a misfire), worse when cold and under load. 150k miles, secondary cats have been deleted but still have primaries. I can get into other parts replaced and addressed, but these readings are an unexplored avenue for me so far.
Vacuum reading at idle is showing 36 kpa (about 10.5 inhg), which from looking through threads seems low. Slowly revving up to 3k rpm in park, readings drop to 27 kpa (8 inhg), which the almighty internet suggests could point to exhaust restriction.
Also - I'm running a 77mm FSP on Tony's tune, and I noticed today when logging that 75% throttle at 2k rpm, when car was bogging down and sluggishly moving forward, boost was almost 10 psi, and this is at about 35 mph. That to me seems quite high, and maybe could be related to all of this?
Yes, low idle vacuum could be due to a vacuum leak, but I've had things apart several times doing repairs and all supercharger gaskets are new, brake booster plug sealed off, vacuum pipe secure etc etc. And absolutely no codes, however did have a MAP reading implausible code on a very long, high speed drive, which could be consistent with a plugged cat. Also - I have intermittent exhaust popping sounds on deccel but am not running Tony's 'pops and sounds' map, and it's intermittent which is even stranger. I'm getting no hiccups, limp home maps etc.
Thoughts? Someone want to give me an excuse to convince my wife it's time for long tube headers?
You very well could have clogged cats. The best way to nail this down is a vacuum/pressure gauge in place of the an up stream O-2 sensor. This is how we test for bad cats.
You should see -0- at idle and 3-5 psi under acceleration but I would not now the numbers for boosted applications so remove the supercharger belt and test it naturally aspirated. If the gauge is showing a lot of pressure you know they are bad...or at least one is.
I have seen cars where at idle it shows 2-3 psi of back pressure because it has no where to go.
You very well could have clogged cats. The best way to nail this down is a vacuum/pressure gauge in place of the an up stream O-2 sensor. This is how we test for bad cats.
You should see -0- at idle and 3-5 psi under acceleration but I would not now the numbers for boosted applications so remove the supercharger belt and test it naturally aspirated. If the gauge is showing a lot of pressure you know they are bad...or at least one is.
I have seen cars where at idle it shows 2-3 psi of back pressure because it has no where to go.
That's great info, thanks! I found so many conflicting numbers of what is 'normal' for back pressure testing since there are so many variables with different engines. I actually have a dual vacuum guage that I bought years ago to tune my old Ducati, I'll see if I can get a threaded adapter and test.
Reason for investigation - car bogs down (smoothly, not like a misfire), worse when cold and under load. 150k miles, secondary cats have been deleted but still have primaries. I can get into other parts replaced and addressed, but these readings are an unexplored avenue for me so far.
Vacuum reading at idle is showing 36 kpa (about 10.5 inhg), which from looking through threads seems low. Slowly revving up to 3k rpm in park, readings drop to 27 kpa (8 inhg), which the almighty internet suggests could point to exhaust restriction.
Also - I'm running a 77mm FSP on Tony's tune, and I noticed today when logging that 75% throttle at 2k rpm, when car was bogging down and sluggishly moving forward, boost was almost 10 psi, and this is at about 35 mph. That to me seems quite high, and maybe could be related to all of this?
Yes, low idle vacuum could be due to a vacuum leak, but I've had things apart several times doing repairs and all supercharger gaskets are new, brake booster plug sealed off, vacuum pipe secure etc etc. And absolutely no codes, however did have a MAP reading implausible code on a very long, high speed drive, which could be consistent with a plugged cat. Also - I have intermittent exhaust popping sounds on deccel but am not running Tony's 'pops and sounds' map, and it's intermittent which is even stranger. I'm getting no hiccups, limp home maps etc.
Thoughts? Someone want to give me an excuse to convince my wife it's time for long tube headers?
You should check your afr readings as well. From all the tunes that i got from tony was really rich.
You should check your afr readings as well. From all the tunes that i got from tony was really rich.
Yeah, I actually ran a dual wideband datalogger after my tune using the rear o2 sensor ports, and AFRs were actually pretty good and consistent.
UPS just brought me a new back pressure tester, hoping to get readings tomorrow. Below is a graph of MAP readings, clearly all over the place and jumping between my idle kpa, and a max of 150 kpa (so a corrected value of 7 lbs of boost). This snapshot was taken while lightly accelerating from a stop, 600rpm to 1500 rpm. There is no way I should be making that high of boost under the conditions, and for how smoothly I was going forward the pressures seem way too out of whack. MAP sensor is brand new Genuine Mercedes, and the old one did the same thing (hence a try at replacement to no avail).
I ran a back pressure test on the passenger side, 0 psi at idle and 2500 rpm, so no problems there. Waiting on the cats to cool down to shove my hand into the drivers side but I'm guessing it will probably be the same as I feel like having only one clogged cat would cause my fuel trims to be off on one bank etc but everything seems to be even all the time. I'll definitely still test it though, but no longer expecting a smoking gun.
Datalogged again and got the car to really bog down badly, see below for the snapshot. Based on the MAP readings, it's saying I hit 11 lbs of boost at 1800-2k rpm, is that possible? Keep in mind I've got a 77mm fixed pulley, but this just seems way too high.
If that boost IS possible - what else should I be logging? (I've verified fuel pressure, IAT, coolant temp, and fuel trims all within normal range).
If it ISN'T possible - What else could cause this? MAP sensor is brand new Genuine Mercedes. Could it be TPS related, or maybe the bypass valve throttle body is shutting closed when it's not supposed to?
P.S. My idle vacuum readings seem normal, I was taking the MAP signal but I hooked a vacuum gauge ahead of the throttle body and it's around 19 inhg at idle.
I ran a back pressure test on the passenger side, 0 psi at idle and 2500 rpm, so no problems there. Waiting on the cats to cool down to shove my hand into the drivers side but I'm guessing it will probably be the same as I feel like having only one clogged cat would cause my fuel trims to be off on one bank etc but everything seems to be even all the time. I'll definitely still test it though, but no longer expecting a smoking gun.
Datalogged again and got the car to really bog down badly, see below for the snapshot. Based on the MAP readings, it's saying I hit 11 lbs of boost at 1800-2k rpm, is that possible? Keep in mind I've got a 77mm fixed pulley, but this just seems way too high.
If that boost IS possible - what else should I be logging? (I've verified fuel pressure, IAT, coolant temp, and fuel trims all within normal range).
If it ISN'T possible - What else could cause this? MAP sensor is brand new Genuine Mercedes. Could it be TPS related, or maybe the bypass valve throttle body is shutting closed when it's not supposed to?
P.S. My idle vacuum readings seem normal, I was taking the MAP signal but I hooked a vacuum gauge ahead of the throttle body and it's around 19 inhg at idle.
You are fixed pulley and I guess if your bypass valve is not opening then yes your boost could be that high.
Well it appears the throttle body did the trick! In the same spot this occurs every morning, it ran nice and strong up the hill, something it hasn't done in a long time. I did go through the TB reset, so will report back if something re-learns poorly. I'm so happy to have smooth power again!
09-07-2020, 07:58 PM
