subtle power loss question at 2400 RPM

[ramonesfreak]

2nd drive of season yesterday and again today I am feeling a power loss at exactly 2400 RPM. I can reproduce it over and over. Only notice when gently accelerating. The tach needle doesnt dop. power loss is very subtle and if i accelerate hard from the start, i dont notice it at all

all new in the past 7, 000 miles/3 years: magna core wires, plugs (Bosch WR-DC+), cap/rotor, coil (MSD blaster)and fuel filter. Car never sees ethanol.

any idea what this could be, or where I should look?

the fact that it happens at exactly 2400 RPMs makes me think this is something specific with spark/timing, however i have no idea what that thing could be

thanks

[ramonesfreak]

Been reading threads here about power loss…my issue is a bit different than what I’m finding here but I’ll keep searching

All I can say is it does not feel like a miss or a stumble. I wouldn’t call it a hesitation. No lugging or chugging. Not really audibly different. Wife couldn’t sense it when I tried to point it out.

Only way I can describe it is the car feels feels more draggy. Almost like taking foot off throttle except there’s no change in speed or drop in rpm. When I accelerate through it, car seems fine

I just checked all my wires to the cap and one popped off a little easier than the others. I put it back on and took off a few times and it does seem to not fit as snug as the others

In the past I have experienced a big power drop at high rpm hard acceleration. Replacing all the above corrected that issue. Otherwise, I’ve never felt anything like it in any car so I’m stumped

[grb008]

Is this CIS?

[spuggy]

Sounds like a flat spot.

Check the advance plate & weights move freely/don't stick or hang up. They can wear - or just need lubrication. Sounds like ignition might be slightly more retarded than it should be there.

Could be the coil (although my only bad coil ever, a vintage Bosch black, was leaking fluid and misfired @ 6000 RPM like a rev limiter).

Or it could be going lean. If you find an incline where you can hold that specific RPM with light load/throttle, does RPM surge up and down without the throttle position or load changing? That's "lean surge".

The idle mixture screw has an effect that extends somewhat further than you may think. If I set %-age CO on my 930 to bottom of spec, it'd idle fine and seem to drive fine - but would lean surge around 2600-2800 climbing the long hill to my house with light load/fixed throttle. I set it to mid-way spec, symptom went away entirely.

Otherwise, you'd probably have to resort to a real-time AFR sensor to see if that's what is going on, and check/adjust your WCP if so. CIS is a balancing act of compromises.

[ramonesfreak]

Yes CIS

Ok thanks for the info I’ll have to start reading my CIS book. Bought the book and gauges last year but have never really needed to use them. So, now’s the time


No inclines steep enough here that I know of so not sure I can try that

[ramonesfreak]

Can anyone give me a reference for learning about these distributors? Is Bentley the book or is there a specific book/manual I can get?

Found this. For reference
https://forums.pelicanparts.com/porsche-911-technical-forum/333640-distributor-service-clean-lube-real-easy-without-removing-pinion-gear.html

[ramonesfreak]

Originally Posted by spuggy

Sounds like a flat spot.

Check the advance plate & weights move freely/don't stick or hang up. They can wear - or just need lubrication. Sounds like ignition might be slightly more retarded than it should be there.

Could be the coil (although my only bad coil ever, a vintage Bosch black, was leaking fluid and misfired @ 6000 RPM like a rev limiter).

Or it could be going lean. If you find an incline where you can hold that specific RPM with light load/throttle, does RPM surge up and down without the throttle position or load changing? That's "lean surge".

The idle mixture screw has an effect that extends somewhat further than you may think. If I set %-age CO on my 930 to bottom of spec, it'd idle fine and seem to drive fine - but would lean surge around 2600-2800 climbing the long hill to my house with light load/fixed throttle. I set it to mid-way spec, symptom went away entirely.

Otherwise, you'd probably have to resort to a real-time AFR sensor to see if that's what is going on, and check/adjust your WCP if so. CIS is a balancing act of compromises.

Can your please tell me what AFR and WCP is?

[spuggy]

Originally Posted by ramonesfreak

Can your please tell me what AFR and WCP is?

WCP = Warm Control Pressure. Measured at the WUR (Warm Up Regulator).

The lower the control pressure, the richer the mixture - the flap in the AFM will move the piston further in the fuel distributor (less pressure == less "stiff" to move), exposing more metering slits for fuel to flow to the injectors. The spec for CCP (Cold Control Pressure - for the warm up cycle) and WCP are a range of acceptable values, which are still within specification.

You may find your car runs better towards the top (or bottom) of the band. At certain temperatures, load or throttle openings...

You can move the pin in the WUR to adjust control pressure; several threads on a modification to make that easier on the board. That, and minor adjustments to the idle mixture (which has decreasing influence/effect higher up the RPM range) are about all there is to adjust with N/A CIS. Probably best to get everything in spec and leave that as last resort/fine tuning.

AFR = Air Fuel Ratio. 14.7:1 with 100% gas (E0) is 1.0 Lambda (stoichiometric). With E10, the stoichiometric ratio is 14.1:1, with E100 it's 9.0:1. All of these read as lambda 1.0 to an oxygen sensor, as O2 sensors measure the amount of unburnt oxygen in the exhaust and generate a voltage showing how close that measurement was to lambda.

Difference between narrowband and wideband is that narrowband can only read a very small range either side of lambda/(stoich), wideband have better resolution, read over a wider range and are heated for faster response after start - and require a controller. If looking to buy a setup, higher numbers are better - for example, the Bosch LSU 4.9 wideband is a more recent design than the LSU 4.2 - but they're both 5 wire sensors (with different plugs so you can't mix 'em up).

Some aftermarket AFR meters read the lambda value and then "convert" to the ideal ratio for pure gas on the display - eg they'll show "14.7" for E10 as though it were E0, even though, if the sensor is reading lambda 1.0, the mixture is actually 14.1...

You should probably know whether the one you use lies to you. Or not. Or just switch it to display lambda and work in that...

If you get ethanol-blended gas in your state, also consider that the blend can vary from zero up to 10% seasonally- they tend to use less (or no) ethanol in for the winter season, because ethanol doesn't light off as well at lower temperatures.

Also, they'll be selling E15 this summer, and stoich for that is 13.8:1.

[ramonesfreak]

Ok thanks. Was going to test my pressures but decided the fuel line there is too brittle to move so I’ve ordered a new one. Once I get that I will measure.

Putting this thread here for future reference
https://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

And this
https://forums.pelicanparts.com/porsche-911-technical-forum/333640-distributor-service-clean-lube-real-easy-without-removing-pinion-gear.html

https://forums.pelicanparts.com/porsche-911-technical-forum/1139977-my-mixture-too-lean.html

[ramonesfreak]

Was doing some reading about exhaust analyzers and see that, at least what you posted Spuggy, the 02 sensor is required. My car has a cat bypass and therefore I assume no place to plug one in


Are there other styles that you could recommend?

[spuggy]

Originally Posted by ramonesfreak

Was doing some reading about exhaust analyzers and see that, at least what you posted Spuggy, the 02 sensor is required. My car has a cat bypass and therefore I assume no place to plug one in


Are there other styles that you could recommend?

Tailpipe sensors seem to read leaner than ones installed in the headers, I think. The one at my local dyno always has, anyway. So I disregard it entirely in favor of one I know the history of, including when I fitted it/how long since I last free-air calibrated it...

If your cat bypass didn't come with an O2 bung fitted, FLAPS stock a weld-in one that any muffler shop should be able to install for you - if they don't also have them on the shelf, which seems unlikely. There are rules as to orientation/where they should be fitted too.

I'd suggest you ensure your ignition is tip-top before diving into CIS mixture - distributors tend not to get their internals looked at for long periods of time and bake all their lubricants into a gummy mess (or a hard paste) when neglected. They should move as freely as possible - because you certainly want all the advance you should be getting.

[ramonesfreak]

Alright thanks for the information

I am having a full suspension refresh done on June 6. I asked the mechanic if he could also splice in a new green wire that I have had sitting here for years and check my ignition timing. He will not have time to disassemble the distributor unfortunately as this shop is backed up getting track cars ready for watkins glen season

If his checking the timing does not correct the issue, I will plan on taking the distributor out and cleaning it next winter. It will take me that long just to build the confidence to do the job

I will also be replacing my magnacore wires with the OEM braided wires with the individual grounds. as well as new plugs, rotor and cap and coil. all should be here in a few days....as its only been 7K miles, everything on my car now should be good for many more years but who knows....

the only issue i ever had with performance was a large dip at hard throttle and high RPM a few years back....changing the above, wires etc.. fixed that...and believe it or not, even totally fixed a cold start surging issue I had lived with for 15 years.....so, not sure what has happened now since the car ran perfect when i parked it in the fall

Ill report back once i get the new wires etc on

[2svnhvn]

Hi, Hmm, you said the car ran perfect in the fall? Does it now have fresh gasoline in it? Perhaps you may have some slight clogging in yhe injection system. I had a similar issue with an older Saab 900 that was CIS injected, I replaced the injectors snd ran fuel injection cleaner through it, and that solved my problem. I’m not saying that is your solution, but perhaps looking into before re-adjusting the CIS.
Good luck,
Pete

[ramonesfreak]

Ran perfect. Always has. Has it been running to its full potential? Not sure because I’ve never driven another SC and have never done any testing but it’s never given me any weird symptoms. I always use ethanol free fuel and used stabil over winter. I did put new fuel in it recently. I also put a bottle of sea foam injector cleaner in it but haven’t had a chance to put miles on.

The biggest tell here is the slight flat spot at exactly the same spot every time….between 2400 and 2600 rpm.

In other cars if I had a bad fuel problem or dirty injector, it ran poorly throughout the rev range and very inconsistently

[pmax]

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies-6.html#post10681789

Nothing in the CIS fueling system is directly RPM driven.

As spuggy said, could be the dizzy.

[ramonesfreak]

yup understood. thats were I am starting once i get fresh wires, coil, plugs cap and rotor installed.

[SCadaddle]

While you are there to replace your cap and rotor, lift up on the rotor to see if your distributor has excessive up and down movement from the worn fiber washer around the shaft at the bottom of it near the gear end. I experienced the same symptoms as you, had a lot of up-down play in the shaft, swapped it out for another "tighter" dizzy and had good improvements in drivability.

[ramonesfreak]

Yup. Will check when I change the stuff myself.

[ramonesfreak]

today put in new plugs, new wires, new coil and new dist. cap.

Couldnt change the rotor because looks like PP sold wrong part....issue here
Rotor questions - 1983


took car out for 30 miles. ran great except the flat spot is still there and same spot

no timing light so cant check that yet

here's the plugs. look ok to me but not sure... not sure why the insulators have so much black on them. that normal?

[917_Langheck]

Look oily and quite fat on the mixture.