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Discussion Starter #1
I've been trying to figure this out for weeks, but I just can't get a hold of it. Maybe someone here can help me. I try to put things here in chronological order, but there's a high chance I'll be missing something. Have to add those later. Here goes.

Symptoms:
-Random but continuous stuttering on cruising RPM. If I pay attention I can see the tacho needle twitch slightly. More strongly present when the ambient temp is below ~20 C (~68 F) but gone when the engine is cold.
-MAF gives very odd readings. Only on idle the value stays reasonable, about 5...8 g/s. On cruising RPM the value jumps up and down from below idle values to way over hundred g/s.
-LTFT is +99.2% on both banks. STFT seems to be normal.
-Also O2 sensors signal voltages jump wildly through the whole scale of 0.1 to 0.9 and some times over that. I've seen readings of 0 and 0.95 and I think they shouldn't do that (except 0 on WOT).
-Also IAT sensor occasionally gives false readings. Or so it seems. Maybe the heat from the engine affects more than I think.
-No DTC. Not even a pending one.

Done so far:
-Replaced the MAF sensor. Old one gave me about 90 mV signal on idle and not much more when revving slightly. Also the throttle response is back to normal with the new MAF sensor. It's a Delphi AF10113. Any experiences on these sensors?
-Replaced spark plugs and wires.
-Tested the EGR valve. Seems to be working and gave the DTC of high EGR flow.
-Tried to locate an air leak, visually and with an starting aid spray. Replaced some deteriorated vacuum hoses on the EVAP system.
-Today I plugged the EVAP at the PCV valve. Still stuttering.

Any ideas what to try next?
 

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Datalog the EEC voltage, and see if it's erratic as well.

You probably have a bad ground wire, probably at the end by the starter.

These are old enough that the cable may be partially rotten, and have green dust coming out of it; that's a sign you need to replace it.

This is the second one this week; there could be a run of these problems... :)
 

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Discussion Starter #3
Too bad this Torque App (on android) doesn't give me that EEC voltage. Need to see if multimeter is of any use on this.

I'm not sure did I get this right. You mean that sensors and / or EEC use the same ground cable as the starter? Or, this ground cable is somewhere near the starter? I'll see what I can find around there anyway.
 

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Discussion Starter #5
Haven't got much time for this, but.. So far only corroded (that green dust) cable I found is by the battery. A Y-shaped cable; one end to the battery negative, one end to the chassis (to a fender mounting screw) an one to a 4-pole connector and onwards to somewhere.. deep. Can anyone tell me what this cable is, which equipment does it ground, and can it cause this problem?

About that datalogging. A datalogger is something more than a simple (cheap) OBD II reader, right?
 

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Haven't got much time for this, but.. So far only corroded (that green dust) cable I found is by the battery. A Y-shaped cable; one end to the battery negative, one end to the chassis (to a fender mounting screw) an one to a 4-pole connector and onwards to somewhere.. deep. Can anyone tell me what this cable is, which equipment does it ground, and can it cause this problem?

About that datalogging. A datalogger is something more than a simple (cheap) OBD II reader, right?
Don't worry about what "it" grounds. Trace it and replace it. It will alleviate future problems if you plan to keep the car. Copper corrosion doesn't stop just because you want it to or because you can't see where the wire leads/terminates at.

As for datalogging, it usually involves saving scanned data to view on a computer. Obviously you can take care of things without one as many do, it will just be more time consuming and $$$ consuming.
 

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Discussion Starter #7
Don't worry about what "it" grounds. Trace it and replace it. It will alleviate future problems if you plan to keep the car. Copper corrosion doesn't stop just because you want it to or because you can't see where the wire leads/terminates at.
Well, yes, it needs to be replaced. I asked 'cause I'd like to know what are the chances for this cable being the culprit, or should I be searching some other damaged parts too. Knowing what these four particular ground leads are used for might help me understand this and the electrical systems of this car in general.

I miss the days when fingers and toes were enough to count the wires in the engine room..
 

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Discussion Starter #8
Update.

I did an emergency repair for that Y-cable I mentioned earlier. It may not be the best of its kind but it should do the job. But this repair did nothing to the problem. There's still that thick ground cable to the engine block. It looks fine to me, but I'll replace that too, later. Oil filter needs to be removed to reach the cable end, so this must wait to the next oil change. Which is soon.

Also, I measured resistances from the block and a couple of grounding points to the battery negative. They all showed 0 to 0.2 Ohms. Does this sound right? I never thought the resistance can go to zero in these cases.. Have to look to it again. There must be more grounding points than those two right behind the headlights. But where? I know two are under the kickpanels in the cabin.

Owner's manual says that there is a relay for EEC in the distribution box, but there isn't. Just an emply slot, not even wired. Can some one explain this?

Battery voltage was 12.25 V at rest. I hoped to see more, but on the other hand the car starts fine every morning. But then again I saw this thread. The battery isn't very old, replaced 04/2011 if the markings are to be trusted. The alternator is replaced at the same time, I believe. It was shining new when I bought the car 11/2012.

And here is something about bad IAC valve and/or O2 sensors.

Comments and suggestions, please?
 

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it takes time to go threw the grounds it is frustrating when doing but in the end it pays off.

what is wrong with the iac and the 02?

is it throwing codes or is it stalling when you first start up?
 

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Discussion Starter #10 (Edited)
it takes time to go threw the grounds it is frustrating when doing but in the end it pays off.

what is wrong with the iac and the 02?

is it throwing codes or is it stalling when you first start up?
Hopefully nothing with the IAC or O2 sensors. It was just one suggestion in a case with similar symptoms. Check the link, if You didn't already.

No codes after I replaced the MAF sensor. Not even a pending one. (Except of course the one initiated by a EGR valve test and erased immediately.)

edit: It has stalled once with no obvious reason, but started normally right after.
 

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I'm interested in which scanner you are using. Mainly to decipher the LTFT readings. Different scanners give different numbers for the same LTFT/STFTs.

What do the STFTs show right when you start up the car when it's cold? Then how do these numbers change as the car warms up in the following minute? How does this number change when it gets to stuttering?

What do the LTFTs show right when you start up the car when it's cold? Then how do these numbers change as the car warms up in the following minute? How does this number change when it gets to stuttuering?
 

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Discussion Starter #12
I'm interested in which scanner you are using. Mainly to decipher the LTFT readings. Different scanners give different numbers for the same LTFT/STFTs.
It's an application for Android, called Torque. Here's a link. My phone is Motorola Droid 4, if that makes any difference. And of course some generic bluetooth transmitter in the OBD II connector. I know it's no way a professional setup, but it's the best (and only) I have at hand right now.

What do the STFTs show right when you start up the car when it's cold? Then how do these numbers change as the car warms up in the following minute? How does this number change when it gets to stuttering?
I've never paid that much attention to it, but I have something.. STFT was +46% when starting up cold this morning as the ambient temp was 12 °C (53 °F). Immediately it begun to decrease steadily to about +10% and then I hit the road. While driving the STFT was low for most of the time, -10..-20%. Occasionally it roamed around zero and when accelerating it increased to about +20%. This is the part that needs more testing, but I believe the symptoms appear when the STFT goes low.

What do the LTFTs show right when you start up the car when it's cold? Then how do these numbers change as the car warms up in the following minute? How does this number change when it gets to stuttuering?
Well this is much easier :) LTFT in +99.2% on both banks all the time. No matter what I do, when I do it, or under what conditions I do it. Also, when KOEO the STFT says "No Data" and LTFT the same +99.2%.

I observed the O2 sensor signal voltages too. At cold start all say 0.1 V. Very soon pre-cat sensors voltages increase quite steadily to 0.9. Post-cat sensors stay at 0.1 V much longer, for the first mile or so. At some point all the O2 voltages start to jump up and down pretty wildly and this is the point when the first stutters appear.
 

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Without knowing in which direction the percentage swings indicate lean or rich, that STFT data is useless. The only thing it tells you is that the air:fuel ratio is not what it should be (which would be 0%). It's one or the other (lean or rich), but which one? Do some research as to which sides (positive or neg. %) mean what, and we might get in the ballpark.

Edit: Nevermind. The torque forums show that a lot of users show the LTFT locked at 99.2, which is what you're showing. That's not right. If you can't trust the LTFT, then you probably can't trust the STFT either.

My gut feeling is that you are experiencing injector(s) being hung open by the EEC, vacuum leak you haven't found yet, or fuel pressure regulator fault.
 

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Discussion Starter #14
How silly of me not looking for Torque forums.. So the LTFT readings are useless. The next thoughts are based on my limited knowledge on fuel injected engines in general and this gut feeling guitar maestro has. Please correct me if.. umm.. when I've missunderstood something.

Let's assume, for a moment, that STFT readings would at least give us a hint of what's going on. A cold engine needs rich mixture to start up and run the first moments. Also, on (near?) WOT EEC orders an extra load of fuel on top of the base dose (and ignores O2 signals). In both cases Torque app shows positive values for STFT, so I presume pos. values means more and neg. values less fuel compared to the base dose. Symptoms appear when STFT readings go low, let's say -10%. Why would it go that low? Is EEC trying to fix overly rich mixture detected by the O2 sensors? If yes, why is the mixture rich?

An injector malfunction: One injector would make trouble on one bank but both STFT readings go very nicely hand-in-hand. Not equal values, but close. Two injectors, one on each bank, failing would be hazardous, I guess.

High fuel pressure: That would affect on both banks equally, just like the situation seems to be. So, fuel pressure reg. fault or clogged fuel return line? I sure hope it's the first one.

Makes sense? Yes? No?

Anyway, I put in an order for a new fuel pressure regulator. Should be at hand on Friday. I hope this works..
 

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Why didn't you just test the fuel pressure beforehand?

As for fueling, the fuel delivery model in the PCM is complicated, and there are too many assumptions. One thing I learned in Logic class, is that once even only one statement is false, any conclusion can be deduced from the original premise. One has to be careful when trying to figure things out when intermediate steps cannot be verified. But generally speaking, the only times the mixture should be commanded rich is: start-up, and when engine load goes over a certain amount while in Open Loop status. At start up the fueling is from pre-set tables in the PCM, ECT value, and an internal timer in the PCM. At higher engine loads (depending largely on whether or not the PCM has detected the Open Loop Flag =1), rich mixtures are commanded from a pre-set table but rely on input from the maf sensor and RPM (holding all other things constant). For PCMs of our cars, when near/at WOT, the STFTs stop reporting fueling corrections (trims), and start reporting commanded lambda. But on most dataloggers, this deviation from stoichiometric fueling is based off of the number "1.00" representing a 14.67:1 air fuel ratio. Lower than 1 is richer, higher than 1 is leaner. The problem is this: TOTAL fueling is not determined by the STFTs correction alone. It is a joint venture between the STFTs (even if it is in OL already) and whatever correction the LTFTs have already stored in their tables. So as you can see, having the LTFTs locked/useless makes trying to deduce anything useless (remember the bit about logic from above), for you cannot determine actual rich/lean fueling (without a wideband O2 sensor) if you don't have real access to both fuel trims.
 

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Discussion Starter #16
I asked around for a fuel pressure gauge to borrow, but none was available. At least no one reported back this quick. Found a gauge set in a nearby store but it costs more than four times the fuel pressure regulator. Now as I think about it, one chance is to go to a hydraulics store and compile an applicable gauge..

I do realize that there is too many assumptions to call it logic in any way. But it didn't cross my mind the fueling could be that complicated..

Oh well, I wait for the new FPR to arrive and get back to this when it's installed and tested on road.
 

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It sounds like an ignition issue, perhaps the ignition coil packs?

Edit:
Check for misfire DTC's
 

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Discussion Starter #18
Replaced the FPR today. The symptoms are now gone, but I'm not very optimistic for it. That stuttering is been vanishing before, when the ambient temp is high. And it's a very hot day here.

It sounds like an ignition issue, perhaps the ignition coil packs?

Edit:
Check for misfire DTC's
Been thinking about the coil packs too, but there's no DTC to be checked.

That MAF sensor still bothers me. Maybe I wasn't clear enought in the first message, but the new sensor seems to be acting up too. (The old one was dead, that's for sure.) This new sensor gives reasonable values in two occasions only: A few minutes after cold start and when accelerating hard enought. WOT isn't necessary, but 1/2...2/3 throttle does it. Rest of the time it's reading is lagging somewhere around idle values or a little higher and making sudden jumps, sometimes even higher than it's possible in the current conditions.
 

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Discussion Starter #20
Or it could just be that data logging app that is not reporting correct maf voltage.
It could be..

I'll check the MAF signal voltage with a multimeter while driving some day. It should be possible with a wire from the MAF signal to the cabin and a good grounding point.

On idle the signal voltage is good, a little less than 1 V. And so is the MAF reading in Torque app, 5..8 g/s and not jumping like it does when driving.
 
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