mdd

In 3rd or higher gear, my car will sputter briefly at around 4000 RPM, clear up, then sputter again at about 5000 RPM. All other engine speeds are fine. The digital boost gauge stays at 0.8 during the sputter. It may really be dropping but the boost gauge is pretty slow. It only does this during WOT and runs fine otherwise. No CEL.

My guess is that I got a bad batch of gas or the formulation has changed due to the winter season. Do the symptoms sound about right for the bad gas theory?

I plan to attach my laptop to log some OBDII data during the event. I expect to see the timing reduced significantly during the sputter event. How much does the ECU reduce timing due to knock?

Other possibilities could be overboost, bad knock sensor, bad sparkplugs/wires.

The logged data should identify overboost. I doubt that's the problem since my car is totally stock with the exception of the muffler bypass.

Will a bad knock sensor cause an error code? How can the sensor be diagnosed?

I assume that a misfire due to bad sparkplugs or wires would result in a CEL. Is that a valid assumption? The sparkplugs have about 5K miles on them.

Any other suggestions or comments to help narrow down the problem?

TIA

Kevin

Plugs, Cap and Wires?

mdd

Is it possible to have misfires due to plugs, wires, or cap without causing a CEL? I assumed that the ECU would detect the misfire and cause a CEL and error code to be logged. Was that a bad assumption?

seege

From my experience misfires have to be tracked down the by the old fashioned process of elimination.I've had misfires from MAS air flow sensor,knock sensor and plugs/cap/wire. Never had seen a CEL until I left the sensor plug off the intercooler.

sonny1

Check plugs,caps,wires,also check mass flow sensor ,check it particulary for oil deposits,if so clean it with carb. spray,cheers,Sonny.

tar6day

Where is the mass airflow sensor? (Where the intercooler attaches to the intake?)

mdd

Thanks for the suggestion about the MAF sensor. That will be easy to check and clean.

I'm going to capture the timing logs today. If the timing is stable through the event, it will eliminate bad gas, knock sensor and overboost as being the possible problems.

tar6day: The MAF sensor is located behind the intercooler, just to the right of center. It's connected between the airfilter housing and the downward turning air duct. Once the intercooler is removed, it should be easy to get at.

tar6day

Thanks! I going to clean it, I have come across some of the same issues.

mdd

Well, I captured the timing at WOT between 3000 and 6000 in 3rd gear. I was hoping to see evidence of the ECU suddenly retarding the timing. The result being the sputter problem. To my surprise, the timing actually advanced slightly during the sputter event. Here is a representative log:

13:26:05 - 45.0 deg
13:26:05 - 45.0 deg
13:26:05 - 45.0 deg
13:26:06 - 44.5 deg
13:26:06 - 39.0 deg
13:26:06 - 40.0 deg
13:26:06 - 40.0 deg
13:26:07 - 40.0 deg
13:26:07 - 38.5 deg
13:26:07 - 37.0 deg
13:26:07 - 33.0 deg
13:26:08 - 32.5 deg
13:26:08 - 29.5 deg
13:26:08 - 22.5 deg
13:26:09 - 21.0 deg
13:26:09 - 18.0 deg
13:26:09 - 22.5 deg
13:26:09 - 15.0 deg
13:26:10 - 13.0 deg
13:26:10 - 15.0 deg
13:26:10 - 14.5 deg
13:26:10 - 14.5 deg
13:26:11 - 25.5 deg
13:26:11 - 41.5 deg

The timing started out at 45. As the RPMs and boost built up, the timing gradually retarded to around 15. The sputter coincided with the 13:26:09 reading of 22.5. I suspect that the timing glitch was a result of the sputter (instead of the other way around).

Does this timing log look about normal? I would have expected the timing to be significantly higher throughout the run. 15 degrees seems extremely low to me.

Unfortunately, the program that I'm using will log only one parameter at a time so no RPM/timing graphs. As a reference point, the 22.5 glitch occurred as the engine was passing through 4000-4500 RPM.

For reference, here is a MAF log from a similar run. Sputter occurred but did not result in any noticable anomoly in the MAF data.

13:28:26 - 14.57 gm/s
13:28:26 - 16.31 gm/s
13:28:26 - 15.75 gm/s
13:28:26 - 16.38 gm/s
13:28:27 - 16.31 gm/s
13:28:27 - 30.40 gm/s
13:28:27 - 62.61 gm/s
13:28:28 - 69.48 gm/s
13:28:28 - 76.14 gm/s
13:28:28 - 84.12 gm/s
13:28:28 - 89.89 gm/s
13:28:29 - 104.39 gm/s
13:28:29 - 119.39 gm/s
13:28:29 - 134.59 gm/s
13:28:29 - 152.98 gm/s
13:28:30 - 180.68 gm/s
13:28:30 - 189.22 gm/s
13:28:30 - 202.68 gm/s
13:28:30 - 215.46 gm/s
13:28:31 - 236.84 gm/s
13:28:31 - 266.41 gm/s
13:28:31 - 284.11 gm/s
13:28:31 - 296.60 gm/s
13:28:32 - 72.67 gm/s
13:28:32 - 19.01 gm/s

I'll clean the MAF sensor and inspect the distributor cap this weekend.

Premier Motorsp

Your problem is very likely the plugs. Put new ones in.

That ignition advance seems low. Is there knock sensor activity? I would guess there should be 18 degrees advance at WOT under boost.

Chris Cervelli
Premier Motorsports

jrgordonsenior

Chris why don't you put 5k on that red car from SB and sell it to me for 66k?

JRG

mdd

Chris: It's funny you mention 18 degrees because that's what the timing was just before the sputter. It then had one reading at 22.5 then down to the 15-13 range. Maybe I am getting some knock resulting in the ECU cutting the timing a few degrees. Is there any way to determine for sure if the knock sensor is activating? I'm just guessing at this point. It would be nice to know for sure.

I'll try the easier things before changing plugs since that seems to be the most time consuming of the various suggestions.

Thanks for the help everyone.

Premier Motorsp

jrgordonsenior:

I don't understand. Maybe you have me confused with someone else?

Chris Cervelli
Premier Motorsports