Weak M50

[RallyD]

'91 M50 5-speed ~200K miles

Car operates great at 0-50% throttle.
Step on the gas beyond that point, performance is the same.
I cleaned the MAF and that helped some of the low speed sluggishness and made the engine more responsive.
Idle is very smooth, revs up to redline with no issues but feels down on power with throttle openings greater than 50%
Yesterday I reset the computer but it still performs the same, flat.
There are no warning lamps illuminated on the dash.

Does anybody have any suggestions on where I should start diagnosing the problem? I was thinking to test the resistance of the TPS potentiometer to make sure it is working properly but other than that I'm not so sure what could be causing the issue.

Has this situation happened to you?

[DaveVoorhis]

Check the fuel pressure.

[Ross]

x2 also check for restricted exhaust.

[bsell]

x2 also check for restricted exhaust.

Seen an Audi Quattro that the owner brought it in cause it started running like a dog. Had no legs on the the top end and sounded kind of quietly funny.

The muffler was full of dry dog food! We had to get a new muffler for the poor guy because all of the moisture in the exhaust had caused the food to swell and stick in place so no shaking the food out. That was the heaviest muffler I've every felt...

Brian

P.S. I've seen a Saab with the same symptoms from a crimped exhaust (half way shut) due to backing into a curb and bending the pipe under the back seat. You really couldn't tell anything was wrong without lifting the car.

[Dave M]

The muffler was full of dry dog food! We had to get a new muffler for the poor guy because all of the moisture in the exhaust had caused the food to swell and stick in place so no shaking the food out. That was the heaviest muffler I've every felt...


OK, I'll 'bite'. How do pack enough kibble in an exhaust to block it?

One determined and creative pranskster?

Mufflers shipped on greyhound :D

Dave

[attack eagle]

MIce/ rats

[fujioko]

I agree with Dave on this. Sounds like low fuel pressure.

[bsell]

OK, I'll 'bite'. How do pack enough kibble in an exhaust to block it?

One determined and creative pranskster?

Mufflers shipped on greyhound :D

Dave

The owner found evidence of mice living in his garage. I can't imagine how they carried the food into a muffler opening about one foot off the ground...

Brian

[RallyD]

Check the fuel pressure.
will do. we just got dumped with some ice here on Long Island so hopefully I can get to it tomorrow.

I'm hoping it is simply a clogged filter. Here's some observations which point to fuel supply.
Driving on highway, 2500rpm, 4th. Punch throttle, mediocre acceleration.
same situation, let off for 2 seconds, punch throttle, better acceleration but fizzles out.
Being as the car was a neglected orphan that I adopted, i'm sure that filter replacement will yield better performance and economy.

i'm also going to get a gauge with a longer hose so i can tape it to the cowl and observe pressures while driving.

thanks to all who replied, i'll let you know how it goes :)

[Karl]

Oh yeah, crudded up plugged filter is a possibility. I can't imagine the original going 200K, but car could be neglected to that extent.

[bsell]

I hope you take every precaution while testing your fuel pressure and driving. I would hate to hear about a fire under your hood or gasoline flooding the interior!:(

If the fuel pressure is found to hold steady under full load, I would recommend taking the muffler out of the equation.

The way the dog food muffle got positively i.d.'d was by disconnecting it and then driving the car. Actually you could tell it was the muffler as at idle the engine picked up a couple hundred rpms after disconnecting the muffler. But of course, the mechanic had to go raise some h*ll without a muffler just to be sure.:D

This might not be easy to do if your exhaust is one piece from the cat back. The Audi's muffler disconnected over the rear axle (flange and reusable dognut). I haven't spent that much time under my '95 525 (thankfully!) to know if the exhaust is one piece or not. Either way, I am sure you can pop it off at the headers and test drive it. But then you won't know if it's the cat or the muffler.

[RallyD]

I hope you take every precaution while testing your fuel pressure and driving. I would hate to hear about a fire under your hood or gasoline flooding the interior!:(


i wouldn't think of putting the fuel gauge in the cockpit. I would put it outside the windshield by the wipers.

I changed the filter and it made no difference.

Then, while driving with the windows down and radio off I heard IT.
Preignition, detonation, ping, whatever you want to call it.

I did a top end cleaning with SeaFoam. Now usually I wouldn't go for something like this (snake oil) but the stuff actually worked. I sucked it into the engine through a vacuum line until the engine died, let it sit for a few minutes, then took it out and flogged it. Looked like Spy Hunter
http://spyhunter007.com/Images/spy_hunter_smoke_screen.jpg

Cleaned out carbon that must have been trapping fuel and heat causing the engine to detonate. The detonation triggered the knock sensor which in turn retarded timing, killing power, and causing the rich fuel puff out the tailpipe on acceleration.

I'm going to give it one more treatment, then change the disgusting plugs (will post pics when I pull them) and then change the crankcase oil and filter.

Its a shame that it has a little more "go" now. I have been ripping off ramps and sweeping turns so much lately that I wiped out the thrust arm bushings.

I do remember there being a dealer procedure using walnut shells in the combustion chamber to remove carbon deposits. Does anyone know any specifics on this procedure? Special equipment other than a nutcracker?

[bsell]

Glad you found your power-robbing demon.

I remember something about walnut shells from my days at the dealership (mid-late '80s) but being an old-fart, I can't remember what the exact deal was. I am sure it had to do with carbon deposit removal but I can't remember which car line and the procedure. The best I can come up with is it had to do with removing the crud off of the intake valves only...

From reading all of the SeaFoam testimonials, I figure the liquid way (20 years newer than walnut shells) may be the better fix as it doesn't leave the hard pieces lying around to burn the valves like walnut shells might. Your only valve-burning concern would come from some flying carbon getting stuck on the valve seat and not moving on in due time...

Brian

P.S. Please tell me you are joking about toasting your arms by having some on-ramp fun time! I am so paranoid that I will rip my front end off by hitting the bumpy twistys they call backroads here in Germany.

[RallyD]

Please tell me you are joking about toasting your arms by having some on-ramp fun time! I am so paranoid that I will rip my front end off by hitting the bumpy twistys they call backroads here in Germany.

oh I wouldn't worry about ripping it off. My bushings are old and worn. I'll get under there this week and see how bad they actually are (probably torn) but in crosswinds the car will try and steer itself. Not so with the other e34.
Oh and the 60mph shimmy is nuts, so I just go faster now ;)

[ryan roopnarine]

they have pressurised devices that you slipstream into the fuel system to remove carbon instead of the walnut shells now. i would think that if one REALLY needed walnut shells in this day and age, they'd go ahead and take the head off instead.

[attack eagle]

MCCC
Mopar Combustion chamber cleaner

beats the pants off that seafoam stuff.