Low Power at Low RPMs

[TX911SC]

New to Porsche...Bought first car a few months ago...'83 911SC. This is the first and only 911 I've ever driven so not sure what "normal" performance is. IMO, car ran very well when I bought it. About 100 miles after purchase, I did some routine preventative maintenance (oil, filter, spark plugs, rotor, cap, adjusted valves, adjusted hand brake, replaced fan/alt belt). Car has continued to run well (I've put another 1,000 miles on it after the initial maintenance).

It may be my imagination, but it feels like the car has recently developed a performance problem. I'll try to explain the problem. The car seems to have lost some power at low RPMs. At low RPMs (1k-3k), mostly in 1st and 2nd gear, the car seems to "lug" or "chug." If I give it too much throttle, it will occasionally pop back through the air box...not a big pop, just a small one (pop off installed). And I'm fairly certain it's through the air box and not the exhaust. After 3,000 RPMs, the performance is absolutely great... The motor winds very fast and smooth through 5,500 RPMs and just keeps pulling very strong and it runs beautifully smooth and effortlessly at 80-90 MPH in 5th.

I recently put a tank of Chevron gas through it to see if that would help (read on the forum that Chevron was good). Made no noticeable improvement. Today, I filled it up with Chevron again and added a bottle of Chevron Tech-tron fuel additive (also read about that on the forum). Drove a couple hundred miles to day and again, no noticeable improvement.

I love the forum and feel like I've been living on it since buying the car (and before). I'm hoping you can help me diagnose my problem. I'm guessing fuel filter, CIS, mixture, vacuum leak improperly adjusted valves ???? Just not sure where to start.

Thanks for your time,
Tom

[ClickClickBoom]

Valve adjust problem? At low RPMs a partially open valve could cause the backfire issue.
Spark plugs?
Start with the things you changed. Plug wires ect.
Eric

[spuggy]

At low RPMs (1k-3k), mostly in 1st and 2nd gear, the car seems to "lug" or "chug." If I give it too much throttle, it will occasionally pop back through the air box...not a big pop, just a small one (pop off installed). And I'm fairly certain it's through the air box and not the exhaust.

When fully warmed up (oil at 176F), or just when cold?

The cold mixture is only ever just right on CIS for about 2-3 weeks twice a year, it seems - I was pleasantly surprised to discover it's "that time again" last night

But CIS isn't EFI; very large & sudden throttle openings result in mixtures that aren't ideal too. Popping back through the intake is lean. Do you get lean surge going up hills below 3000 RPM at a steady throttle opening?

[Josh D]

Have you checked your ignition timing? Vacuum advance working?

Doing the other work you did could have disturbed a vacuum line. They get brittle and can crack when disturbed creating a vacuum leak.

[DRACO A5OG]

If you have a Cat, after a long drive, see if it glows red, if so then you found the culprit.

[TX911SC]

Originally Posted by spuggy

When fully warmed up (oil at 176F), or just when cold?

The cold mixture is only ever just right on CIS for about 2-3 weeks twice a year, it seems - I was pleasantly surprised to discover it's "that time again" last night

But CIS isn't EFI; very large & sudden throttle openings result in mixtures that aren't ideal too. Popping back through the intake is lean. Do you get lean surge going up hills below 3000 RPM at a steady throttle opening?

I always let the car idle until the temp gauge reads above the large white area before leaving the driveway, then I take it easy (below 3,000 RPMs) until it's up to normal operating temp. The popping is worse when it's cold, but still occurs even when at normal op temp when I give it lots of throttle at low RPMs.

Your comment about CSI not likeing "sudden throttle openings" is interesting (and humorous :-). You're right. At low RPMs it doesn't like it and that's when it occasionally pops. But over 3,500 RPM, WOT is great. Regarding your comment about uphill below 3,000 RPMs the power is low.

So bottom line.....You think I'm running lean?? Why would that lean condition only be present at low RPMs? If that's the cause, I'll focus there and learn how to adjust the mixture....or whatever you do with CSI.

Thanks for your help

[TX911SC]

Originally Posted by Josh D

Have you checked your ignition timing? Vacuum advance working?

Doing the other work you did could have disturbed a vacuum line. They get brittle and can crack when disturbed creating a vacuum leak.

I've never messed with the timing and don't know how to do it, but I will break open the Bentley and check it out. I'll also look for the location of the vacuum lines and check to see if one is off or broken. Thanks for your help.

[TX911SC]

Originally Posted by DRACO A5OG

If you have a Cat, after a long drive, see if it glows red, if so then you found the culprit.

I promise I won't always be this stupid about 911s, but I'm not sure if I have a cat or not. I have a long (~3 feet) what I thought was the muffler that is mounted just in front of and parallel with the rear bumper (see pic). The other 2 things that look like they could be cats I thought were the heater boxes.

[K Sykes]

Cat would be between the muffler and the heat exchangers, running parallel to the direction of travel on the drivers side.

[911s55]

Originally Posted by TX911SC

I always let the car idle until the temp gauge reads above the large white area before leaving the driveway, then I take it easy (below 3,000 RPMs) until it's up to normal operating temp.

While this is not likely the culprit it is also not recommended and is not how these cars should be treated after start up. Start the car, drive immediately under or around 3,000 RPM until at operating temp, then have at it.

David

[TX911SC]

Originally Posted by K Sykes

Cat would be between the muffler and the heat exchangers, running parallel to the direction of travel on the drivers side.

Definitely have a CAT...just behind the left rear tire. Thanks for your guidance. Don't know why I didn't make a note of it when I was adjusting the left side valves. Will go for a drive tomorrow to check to see if it glows red as Draco mentioned above.

[spuggy]

Originally Posted by TX911SC

The popping is worse when it's cold, but still occurs even when at normal op temp when I give it lots of throttle at low RPMs.

You always get a momentary lean condition on just about any system when you allow more air in (by opening the throttle); EFI compensates quickly enough that it's not an issue, carbs often have an accelerator pump for the same reason.

Older CIS versions don't really provide compensation, although, some later CIS variants allow provide additional acceleration enrichment via vacuum control, I believe. But now we're getting into "I read it on the internet" for me.

Your comment about CSI not likeing "sudden throttle openings" is interesting (and humorous :-). You're right. At low RPMs it doesn't like it and that's when it occasionally pops. But over 3,500 RPM, WOT is great. Regarding your comment about uphill below 3,000 RPMs the power is low.

OK, a "lean surge" is when you hold a fixed throttle position under a constant light load (like climbing a hill @2800 RPM), and the RPMs don't stay constant, they hunt faster/slower, by about 200 RPM or so.

So bottom line.....You think I'm running lean??

A backfire through the intake is either too lean or too much advance, if valve timing is correct. Most people generalize...

Why would that lean condition only be present at low RPMs? If that's the cause, I'll focus there and learn how to adjust the mixture....or whatever you do with CSI.

Thanks for your help

Overall mixtures are controlled via the cold control pressure (CCP) or the warm control pressure (WCP) in the warm-up regulator (WUR). Which, despite its name, controls mixture for all temperatures. Straight forward, eh?

However. CCP (when cold) or WCP (when hot) controls mainly the wider throttle openings; the idle mixture ALSO has an affect on the mixture; this affect diminishes the higher the RPMs rise (and is mostly all gone by 3-3500 RPM).

For example, if the idle mixture on my 930 is set leaner than factory spec, it seems to mostly run just fine - except it will lean surge climbing hills with no/light load. Regardless of what the WCP is set to. And it'll tolerate less enthusiasm with the loud pedal when cold before spitting back into the intake.

Stock WUR's aren't adjustable; you're supposed to get them rebuilt if they go out of spec (and checking pressure is critical). You can tap the pin to adjust the CCP (which also has a knock-on affect on the WCP), or there is a modification that can make the CCP adjustable (getting a bolt in the pin). It's also possible to make both CCP and WCP individually adjustable, if needed (Brian Leaske is awesome).


I'd suggest to check 5 things, after you double-check everything you recently did, heh:

Ignition timing at full advance
Ignition timing at idle
Cold control pressure
Warm control pressure
Idle CO2

If the advance plate is not fully retarding (returning), then you'll have more advance at low RPM than the motor wants/can use.

If you have a CO2 sensor (and thus the US lambda system), this system should be functional; this system regulates WUR pressure via the O2 sensor, a brain, and a frequency valve to bleed off fuel pressure to the return (thus richening the mixture) and will run lean without it, unless it is adjusted to compensate. If you have this, there's a test point; you check operation with a dwell meter, like points...

[juanbenae]

was the fuel filter replaced in your efforts? seems that might affected more on the high rpm range, but with all you did i thought id ask?

[TX911SC]

Originally Posted by spuggy

You always get a momentary lean condition on just about any system when you allow more air in (by opening the throttle); EFI compensates quickly enough that it's not an issue, carbs often have an accelerator pump for the same reason.

Older CIS versions don't really provide compensation, although, some later CIS variants allow provide additional acceleration enrichment via vacuum control, I believe. But now we're getting into "I read it on the internet" for me.



OK, a "lean surge" is when you hold a fixed throttle position under a constant light load (like climbing a hill @2800 RPM), and the RPMs don't stay constant, they hunt faster/slower, by about 200 RPM or so.



A backfire through the intake is either too lean or too much advance, if valve timing is correct. Most people generalize...



Overall mixtures are controlled via the cold control pressure (CCP) or the warm control pressure (WCP) in the warm-up regulator (WUR). Which, despite its name, controls mixture for all temperatures. Straight forward, eh?

However. CCP (when cold) or WCP (when hot) controls mainly the wider throttle openings; the idle mixture ALSO has an affect on the mixture; this affect diminishes the higher the RPMs rise (and is mostly all gone by 3-3500 RPM).

For example, if the idle mixture on my 930 is set leaner than factory spec, it seems to mostly run just fine - except it will lean surge climbing hills with no/light load. Regardless of what the WCP is set to. And it'll tolerate less enthusiasm with the loud pedal when cold before spitting back into the intake.

Stock WUR's aren't adjustable; you're supposed to get them rebuilt if they go out of spec (and checking pressure is critical). You can tap the pin to adjust the CCP (which also has a knock-on affect on the WCP), or there is a modification that can make the CCP adjustable (getting a bolt in the pin). It's also possible to make both CCP and WCP individually adjustable, if needed (Brian Leaske is awesome).


I'd suggest to check 5 things, after you double-check everything you recently did, heh:

Ignition timing at full advance
Ignition timing at idle
Cold control pressure
Warm control pressure
Idle CO2

If the advance plate is not fully retarding (returning), then you'll have more advance at low RPM than the motor wants/can use.

If you have a CO2 sensor (and thus the US lambda system), this system should be functional; this system regulates WUR pressure via the O2 sensor, a brain, and a frequency valve to bleed off fuel pressure to the return (thus richening the mixture) and will run lean without it, unless it is adjusted to compensate. If you have this, there's a test point; you check operation with a dwell meter, like points...

Spuggy -- A quick "Thank You" for your comprehensive response to my request for help. Your knowledge of 911/930 systems FAR exceeds mine, so I'm going to have to sit down with your response and the Bentley manual to try and understand (and act on) all the info you've provided. I'd like to reserve the right to chat with you more after I've run this to ground. But again, thanks.

[TX911SC]

Originally Posted by car 311

was the fuel filter replaced in your efforts? seems that might affected more on the high rpm range, but with all you did i thought id ask?

311 -- I did NOT replace the fuel filter. I bought one from Pelican at the same time I bought all the other parts for the initial maintenance, but I didn't install it because I read that I should replace the accumulator at the same time. So, I've put off installing the new fuel filter until I get the accumulator. As I mentioned in the beginning, I was wondering if the fuel filter could be the problem. I didn't realize a bad fuel filter would show up more at high RPMs. So, thanks for that info. Since it performs great above 3500 RPMs I guess it's not the fuel filter. Again, thanks.

[al lkosmal]

CIS Primer for the Porsche 911

This may be of help to you. A lot of CIS info regarding operation, testing and troubleshooting. Nice systemic approach to understanding and troubleshooting your system. Could save you a bit of time.

regards,
Al

[E Sully]

The CIS primer gives great info on the fuel system. A fuel gage setup is important to getting the cis in spec. Also make sure your distributor is in good condition.
distributor lube...
Distributor service (Clean and lube) real easy without removing the pinion gear!
Distributor Maintenance

[techman1]

When cold, look over the intake closely, looking for any loose connections.
If possible, have someone start it when cold, and blip the throttle, while you carefully look for air leaks. NO loose slothes, watch that fan!!

It is possible when it warms up the expansion of parts is closing your air leaks, so it will run better.

[spuggy]

Originally Posted by TX911SC

Spuggy -- A quick "Thank You" for your comprehensive response to my request for help. Your knowledge of 911/930 systems FAR exceeds mine, so I'm going to have to sit down with your response and the Bentley manual to try and understand (and act on) all the info you've provided. I'd like to reserve the right to chat with you more after I've run this to ground. But again, thanks.

You're more than welcome. Al posted a link to a great guide by Jim; for practical purposes, that's probably much more useful than Probst's book.

Be careful with specs from Bentley, although good with process/test procedures etc they've been known to quote actual specifications incorrectly - suggest you find your motor type #'s, WUR part # etc. and get someone to post those from the factory documentation.

I suggested checking you're in basic spec Just Because you're wasting your time doing much else if any of those are wildly out.

But as this suddenly developed, chances seem high it's either because the weather changed (no, really - although your later CIS should, if working correctly, be able to compensate), or because something came unplugged, blew a fuse or just plain stopped working.

Did you check for a vacuum leak yet?