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Old 09-08-2018, 05:49 AM   #1
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Low RPM stutter and wobbly shifter

Hey all,

Got two issues that are driving me nuts on my 99 5speed w/145k on the clock.

Issue 1 is that below 2000 rpm I get a stutter. If I keep it on the boil above 2k, then it runs fine, no matter throttle position. However I get an inconsistent stutter below 2k. No CEL the majority of the time, the others it's gone before I can get to the house and check it. Have not done any work yet other than check my MAF (sparkly clean) and order stuff to change coils and clean TB when I get some time off.

Issue B, stock shifter has a lot of side to side play. I think this is leading to some difficulty shifting. Think about 3/4 inch side to side when in gear.

I have the funds to fix it right, these getting fixed would transform the car into a reliable machine.

Thanks in advance

A

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Old 09-08-2018, 06:43 AM   #2
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Pardon me being Captain Obvious, but a stutter is a combustion issue...either fuel or spark. Start with the simplist things; change your plugs and try some Techron. If that doesn't solve the issue, keep working the problem backwards. Without a CEL, it's just guesswork, and I'll differ to those who have better guessing outcomes. Lol.
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Old 09-08-2018, 07:54 AM   #3
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This is a silly question, but why are you driving below 2k rpm? I rarely drive below 2800 rpm. Keeping the revs up keeps the splash lubrication going for your IMS bearing. The only time I'm below 2k is for a split second in 1st gear!
Sorry, couldn't resist. I'll defer to someone who's actually helpful now!
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Old 09-08-2018, 09:11 AM   #4
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More than likely the easiest thing to do is downshift. I know my car does not like to sit below 5% throttle at low RPMs. I try to keep mine to 3k+ minimum. Otherwise the compression gets the better of the motor and you are lugging it, which will cause it to “stutter”. Regarding your shifter, that is a relatively easy DIY to swap and not too expensive. They can get sloppy over time.

It is possible there are some other things going on relative to your motor and fuel/air but most likely you just need to get the revs up. You can still check for CELs it should still store. If you regularly drive at that low of rpms you can have carbon build up, as mentioned check the obvious (air filter too) and try some fuel system cleaner. Then go from there.

Last edited by Geof3; 09-08-2018 at 09:14 AM.
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Old 09-09-2018, 04:04 AM   #5
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Originally Posted by MWS View Post
change your plugs .
Plugs have the easiest time at low RPM. Although the CR may be quoted at 11:1 at low throttle openings the actual CR is much lower. Plugs fail at higher RPM and high throttle openings when the actual CR is higher and therefor more compressed air/fuel between the plug gap making it harder to jump the gap.

Probably more like the MAP or airflow sensor not set correctly at the bottom RPM range.

Does it improve when the car warms ups ?
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Old 09-09-2018, 05:09 AM   #6
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I replaced the plugs due to leaking tubes a few months ago, problem didn't start until some time afterwards.

It does seem to better when warmed your, but occurs whether the MAF is plugged in or not.
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Old 09-09-2018, 06:06 AM   #7
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I owned a Buick Park Avenue that started misfiring at mid-throttle. If I lifted the throttle the engine wouldn't fire at all. It got so bad that I had to give it full throttle on the highway to keep it going (I was on my way to Chicago from Boston). When I got to Chicago I had a workshop try to diagnose the misfires. They kept focusing on the injectors and I kept telling them to change the spark plug wires first. After several hours of them mucking around they finally followed my instructions and bingo, misfires gone.

The problem was cracked insulation on the wires. At low throttle settings and at full throttle the compression high enough to allow the plugs to spark, but at mid-throttle the compression was low enough that the spark went through the cracks instead of through the plugs. If you have ever owned a car with a vacuum gauge you will know when the compression is low. High vacuum = low compression.
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Old 09-09-2018, 10:15 AM   #8
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Quote:
Originally Posted by marsheng View Post
Plugs have the easiest time at low RPM. Although the CR may be quoted at 11:1 at low throttle openings the actual CR is much lower. Plugs fail at higher RPM and high throttle openings when the actual CR is higher and therefor more compressed air/fuel between the plug gap making it harder to jump the gap.
This is one reason I love this forum...being wrong, for only then do I learn something.

I should state that I'm not a mechanic (more of a home tinkerer), and the opinion I offered was based on other combustion engines (lawn mowers, etc) I've worked with that have a stutter at idle and I've found dirty/fouled plugs to be an issue. I'm not doubting your statement, but it would seem to me that the "misfire rate" at low RPMs would make a stutter much more pronounced, versus at a high RPM where the occasional misfire wouldn't be as detectable, or at least audibly. Again, I actually enjoy being wrong, so if you would be so kind as to offer a "Googleable" (LOL) starting point, I can start my education.

Actually (after re-re-rereading your post), it start to makes sense regarding density and gap as related to plug failure (or at least firing), but to me it still seems that once a plug had started the failure process that it would be most detectable at low RPM even though the CR is much lower. Grrrrrr.

Also, FWIW, (as stated in a previous post) I've also had an issue with a vehicle that had a misfire that plug leads were responsible for....changing them out solved my issue.
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Old 09-10-2018, 03:57 AM   #9
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A bit of 2 stroke history.

In the old days of 2 stroke and points, plugs had a real hard time. At low power settings, the plug electrodes did not get hot enough to burn off the oil/carbon on the electrode. Lawn mowers/bikes ran great. Next time you tryed to start it, nothing. The problem with points is that the spark rise time was slow. So slow that as the voltage rose, it would leak down through the carbon on the electrode and never jump the gap. Now take your neighbour that ran his mower full taps. Ie hot plugs. He never had a problem starting as his plugs were hot enough to burn off the carbon. The correct electrode/porcelain should look white to about 1/3 of its length. Ask any 2 stroke racer and he will tell you.

Now roll on the CDI - Capacitive discharge ignition. The rise time of the spark is very very high. So high that it will relieve its energy where ever it can. Down the electrode carbon as well as the spark gap so now the old fowled plug will still fire and in most cases the motor will run. Once it warms up the situation gets better.

I had a mini may years ago (a real one) and I made a CDI for it. One misty morning I started it and while running I heard this loud Clac Calc. I opened the bonnet. Due to the heavy mist the plugs were wet. No problem for this CDI , it had a nice fat blue spark running down the outside of the plug while the car was still idling perfectly.

TCI Transistor assisted ignition is a bit between the 2. Not sure what Porsche uses.

In most cases a good thrashing of the motor will burn of the carbon and return the plugs to normal.

PS if you always tootle around in your car, you need a hotter plug, if racing a colder one.

The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine, normally between 500 and 800 C (932 and 1,472 F).

The heat range of the plug helps keep the tip at the correct temperature and burn of the carbon.
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Old 09-10-2018, 10:09 AM   #10
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In most cases a good thrashing of the motor will burn of the carbon and return the plugs to normal.
Which is one reason a Porsche motor (any really) usually runs better after a track day. And illustrates part of the point of not running these motors at low RPMS...

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