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Rough Idle, 911 3.2, ~50 RPM variance

Hey all,

At this point I think I've read every thread out there regarding engine "calibration." I'll get right to the point:

Symptoms:
- When engine is not fully warmed up, idle will fluctuate between ~880 and ~810 RPM.
- Initial cold start is fine; engine stays at ~1100 RPM before easing down to 880.
- Symptoms are gone when engine is fully warmed (30-45 minutes of driving, engine oil temperatures with in-cabin reading of between 165-180oF or above)
- Symptoms steadily decrease while engine is warming up.
- When coming to a stop (or letting off the throttle) out of gear, the engine speed is prone to decreasing below ~880 RPM while engine is warming up (see above notes about what I considered "warmed up"). Becomes less and less of a problem as engine gets warmer.

Vehicle Specs:
- Vehicle in question is an '88 911 3.2
- Vehicle has SSI's, M&K 2-in 1-out muffler, and a Steve Wong 93 octane chip
- All other engine components are stock (to my knowledge)

Recent engine maintenance (by me, within past 500 miles):
- Ignition rotor replaced
- Ignition cap replaced
- Idle control valve replaced
- Fuel pump replaced
- Redline SI-1 treatments

Recent engine maintenance (at 'Shop A', within past 500 miles):
- Valve adjustment
- Oil change

Recent engine maintenance (at 'Shop B', within past 3000 miles):
- Valve adjustment (I think it was done improperly, with engine warm, as was loudly ticking within a few thousand miles; recent adjustment has solved it)
- Spark plugs replaced
- O2 sensor replaced

Historical maintenance:
- Engine rebuild sometime near 100k miles; compression shows low single digit figures.
- Spark plug wires replaced
- Ignition coil replaced

Recent Testing/Calibration:
- *Using an Innovate LM-2 wideband AFR sensor setup*, idle AFR has been set to 14.2 (corresponding to ~.7% CO) with O2 sensor unplugged and in-cabin gauge temperature reading approximately ~180oF. AFR holds quite steady, occasionally blipping to either 14.3 or 14.1, but infrequently. This AFR was brought down from an idle AFR of 15.1 or so.
- Set base idle speed (jumpered test ports) to ~880, calibrated using clamp on digital tachometer. Base idle was set much too high after richening the mixture
- Confirmed proper function of idle switch using multimeter (continuity vs. no-continuity)
- Confirmed either minor or no vacuum leaks; when oil cap is removed, idle becomes *very* rough.
- O2 sensor looked blackened when pulled (nothing remarkable)
- Cap on backside of DME did not appear to have been removed; I did not check to see if it or if the fuel trims (alterned by the knob behind the cap) had been adjusted.

Questions for those who may know:
- Can the CHT sensor (2-wire, mind you) partially fail?
- At what temperature (oil, or after how long driving) is the CHT sensor considered "warmed-up?"
- I have not pulled the spark plugs again yet; I have a set on hand and was going to replace, but any thoughts on this being an ignition issue?
- Do I need to go even richer, considering E10 (10% ethanol) should have a lower AFR target? These cars were calibrated pre-ethanol blend, so I'm not sure if I should take that into account. Also, the 14.2 idle AFR I targeted puts it on the slightly leaner side of spec...thoughts? Again, the problem only manifests when the engine is warming up, not already warm, so I'm hesitant to go further with idle AFR adjustments.

General Thoughts:
- Am I being too picky? The car is running better than it ever has, but I feel like I shouldn't be having the above mentioned issues. Everything that has been changed or has been done has steadily improved things; I'm just not sure if I've reached the endpoint, as I don't know if what I'm experiencing is normal. Again, everything is perfect if the car is fully warmed up. It feels perfect. It seems to be only the first 30-45 minutes (longer for cold days, shorter for warmer) when things are rough.

Thanks,
Matthew

Last edited by mmahon04; 02-10-2014 at 09:06 AM..
Old 02-10-2014, 09:00 AM
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Check out #5 in this thread.

Car will not idle, frustration builds!
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Old 02-10-2014, 10:07 AM
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I have a similar "problem", sometimes when warming up my idle will drop and stick right at 880 but usually it falls somewhere slightly lower, however once the car is warmed up it eventually transitions to occasionally sticking a little bit above 880. I'd say I have a variation of + or - 50rpm, but then sometimes it stops right on 880 still.

I have also done similar extensive work to try to solve it. If I set my base idle then I can nail it right on 880 and go rev the car and everytime it falls and sticks on 880. But then when I'm out driving around I still get the varying behavior.

I have a Steve Wong 93oct chip too and Steve recommended I set my mixture to 14.2-14.4 so that's what I did with my LM-1, although mine blips from 14.1-14.5. I've tried many different base A/F ratio settings and none seem to affect this issue.

Also, I don't believe this is a symptom of a failing CHT, actually I have also replaced my CHT with a new 3 wire version and that didn't affect it.

I'm just starting to accept that maybe this is normal?

Last edited by Hunt3R; 02-10-2014 at 03:16 PM..
Old 02-10-2014, 03:13 PM
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If I could tune my MegaSquirt on my 3.2 (3.4) to only have only a 50 RPM fluctuation at idle, I'd be ecstatic.

Most endure 200-300 fluctuations.

There are lots of threads discussing this if you search for surging, hunting, oscillating, etc., idle issues with the Carrera 3.2.
Old 02-10-2014, 03:37 PM
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put a stock chip back in for test purposes and see if things improve.
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Old 02-10-2014, 03:54 PM
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Follow up for anyone interested:

1) Per Bob's link (post #2 in this thread), I tightened oil cap as far as I felt comfortable. Idle has possible started improving a bit earlier when driving (not needing to warm up as long now), but this could purely be placebo. I might order a new seal after I've completed the tests below...
2) This weekend I plan on checking CHT sensor resistance, O2 sensor resistance, and AFM temperature sensor resistance. Will report back.
3) I'm starting to think this could be related (partially or fully, or not at all?) to a leaky injector. I've always had a hard time having the car fire right up after I kill the engine and immediately try to turn it over again. I plan on checking fuel pressure leakdown in the near term.

Will keep posted. I've always hated threads that have no original poster follow up.
Old 02-12-2014, 05:42 PM
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Thanks for the follow up, I hate that too.
Old 02-12-2014, 06:02 PM
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More reading

Starting to think more heavily about the idea that it could be a leaky injector. Read a thread here, and can't help but think a lot of the thoughts/symptoms sound possible:

Quote:
Originally Posted by DRACO A5OG View Post
Once removed, the old ones were clean at the tips probably because of the SeaFoam I used, but the plastic tips were reddish brown in color and appeared to be varnished. (pics to follow) There were bunch of carbon deposit surrounding it.
Quote:
Originally Posted by DRACO A5OG View Post
It has been 3 days, and no leak, instant start ups, fantastic runs via throttle response, smooth idle. She even sounds better.
...and the key quote here:

Quote:
Originally Posted by DRACO A5OG View Post
But the Fuel Injector with maybe help from the NGKs is keeping the trickle down to a minimum and only to 800RPMs . Prior to the swap out she would trickle down to 750 RPMs, a forced lean due to rich Air Fuel mixture at idle.
I do quite a number of short trips, so I am wondering if I am getting a varnish buildup that Redline would be unable to resolve. The 'RPM trickle down' sounds awfully appropriate for what I experience; nothing as low as the 750 RPM he experiences, but it does trickle down then does pop back up, just to trickle down again. Warm/hot starts are indeed somewhat tough for me (no impossibilities yet), which can indicate fuel pressure leakdown. Idle is also more sensitive to injector state; off idle the volumes of fuel and air can mask 'minor' flow issues. Car has 119k miles on it, so the injector wear possibility is starting to come to forefront of my mind.

Still plan on doing above mentioned tests this weekend. Fuel pressure leakdown might get added to Saturday to-do list, but this will require some research.

Last edited by mmahon04; 02-13-2014 at 08:28 AM..
Old 02-13-2014, 08:23 AM
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Curious to know what you mean by "warm/hot starts are tough"?
Old 02-13-2014, 08:42 AM
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Hunter,

Say I have the car completely warmed up (have been driving for 45 minutes, oil temp at least 180), and I shut the car down. If I try to turn the car back on immediately or within the next several minutes, it takes longer to crank and when it catches, the RPM's don't immediately jump to 880, they seem to slowly build from a low base (e.g. instead of cranking, 0 RPM, then 880'ish RPM when it catches, it will catch at 300-400 RPM, then build up to 880). Starter is fresh and battery checks out, and again, this only happens on warm starts within shorter time period after shutoff.
Old 02-13-2014, 10:11 AM
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The hot start issue sounds like a leaky check valve (or leaky injector) to me.... the hot engine boils the fuel in the rails since the rails cannot hold pressure. Get a fuel pressure gauge and check according to the WSM: Operate the fuel pump for one minute and then shut it down. You need to see at least 1 bar 20 minutes after the fuel pump is shut down. A leaky injector or bad check valve will become obvious pretty quickly

ingo
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Old 02-13-2014, 10:30 AM
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ischmitz,

That was exactly my thinking. Fuel pump (including check valve) is new, so that leaves a leaky injector as a possible culprit. Considering a rough idle can be a symptom (as well as possible warm/hot start issues), this is exactly why I am leaning this way now.

Thanks,
Matthew
Old 02-13-2014, 10:47 AM
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Careful, even a new fuel pump could come with a bad check valve out of the box. I wouldn't discount that.
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Old 02-13-2014, 01:47 PM
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Follow up,

Yesterday afternoon I checked out a few of the relevant sensors to my issue. Engine was still feeling rough when completely warmed up, but instead of idle RPM's drooping to ~810 (with few moments slightly below), they seem to fall to ~830 or so, at most. So, issue is present with car warmed up; its just slightly less so.

Tests run:

1) ~.76 VDC between the male prong and chassis ground off the O2 sensor. Bentley manual states ".1-.9 VDC fluctuating." I think the sampling rate on my multimeter is too slow to pick up the rapid fluctuations (I was getting a slightly different number every second or two, but all .76 VDC +- .03 VDC). Looks like I'm good there.

2) With engine oil temperature reading slightly above 180oF (cabin gauge) and after driving for ~35 minutes, I shut the car down, and within 3-4 minutes had leads on the CHT sensor. Initial reading was ~85 ohms (per 240-18 in Bentley, this corresponds to head temp of ~265oF), which began to rise. Several minutes later with the engine still off (oil temperature having now fallen to slightly below 180oF, gauge indicated), CHT sensor spec'ed out at 180 ohms (roughly equal to head temperature of 212oF). I have not tested with engine stone cold yet today, but do not plan to do so as things seem to be in line and I have no cold start issues.

Also did some more research on fuel injectors:

1) Spoke to Rob at Motor Man Fuel Injectors (recommended per this forum). Very helpful, he confirmed that the -0158 injectors (the stock ones) are prone to leaking at multiple points. This also aligns with WitchHunter's statements of the -0158 being a known problem injector.

Going forward:

I went ahead and ordered the -0360 injectors from Motor Man, as WitchHunter will be ~$22.00 for refurbishment of my stock injectors apiece and still have possible problems, while I can order refurbed, non-problematic injectors from MotorMan at ~34.99/ea. A fuel pressure test kit was going to run me north of $100, so in this case I will throw some parts at a problem because I feel I've adequately tracked it down. Hopefully they arrive by this next weekend. I'll follow up with everyone then.
Old 02-15-2014, 07:27 AM
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Left Bank Injectors Installed

Hey all,

I went ahead and installed the replacement injectors in the left bank of the engine. My car has backdated heat, so between that and not needing to remove an A/C compressor, airbox, and AFM, the left side ends up being very accessible. I didn't feel like eating up several hours either, so I've only done the left bank thus far.

Notes on doing the job:
1) 2x 10mm bolts (not nuts) hold the fuel rail to the intake manifold.
2) There is a single 10mm bolt holding a padded clamp (with a wire harness run through it) to the fuel rail.
3) Removing the clamp assembly that holds the CHT sensor connection (among the 2 others) really frees up some room to work. In fact, I think you'll have to remove it to get enough room around the rear of the fuel rail to work. 2x 10mm *bolts*, by the way.
4) Connections of fuel line to fuel rail are both 19mm. Be careful, both are *threaded* the standard way (left to loosen), but you will loosen the rear connection the opposite direction from the front; i.e. pull the 19mm wrench toward you to loosen the closest connection, push away to loosen the rearmost connection in back of engine bay.
5) Twist the injectors (after lubing the seals) into the rail. Pushing them directly in is fruitless.

Notes on effect:
1) Car barely has/no longer has any issues with hot starts
2) RPM's do not dip below 880 when letting off throttle. No more coming to a stop, put in neutral, engine drops way below 880 to raise back up again.
3) Idle still has a bit of fluctuation, but is perhaps only 30 RPM now at most.

Other observations:
1) Looking at the fuel injectors I pulled, there was some obvious dirt/black material on the pintle of each injector. Definitely on the outside vertical area of the pintle (not even in the spray zone), so I do not think it was having an effect. Just something I noticed.
2) The injectors themselves, 1 in particular, had a reddish varnish at two particular spots on them: right below the electrical connector to the injector, and right above the O-ring for the pintle area. The varnish/buildup looked to me like the residue of a liquid that had evaporated, possibly repeatedly, and was in such a pattern to look like a leak (broader at top of injector, slimming down along body of injector). I had read (see above) that another forum member had seen some reddish varnish on their injectors as well, speculating that it was the remnants of evaporated fuel; I think I can confirm that at this point.
3) Some people seem to be keen on picking up the -0364's over the -0360's, the reason being that the pintle is metallic rather than plastic, this possibly lending itself to less heat soak and less varnish buildup, The pintle's in all of mine thus far show no varnish after 119k miles on stock injectors, so if anyone is waiting to pick up -0364's over -0360's because they are supposedly "functionally better," I would have to disagree.

I would say my symptoms are 65%+ alleviated, based on my few hours driving today. My injectors are certainly worn on the right bank, and I'm thinking I still have a leaky one over there somewhere. Looking forward to changing out the next three.

Last edited by mmahon04; 02-21-2014 at 06:46 PM..
Old 02-21-2014, 06:43 PM
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Shade Tree Me

There are times on the Forum when I feel completely inadequate as a shade tree mechanic and a man.

....this is one of them.

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Old 02-22-2014, 05:00 AM
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