How to diagnose and resolve engine issues using Durametric.

[jb92563]

I recently had an AOS failure and proceeded to replace the AOS.
After the replacement the engine idles poorly and has hesitation at various throttle settings it seems.
I cleaned the TB and got the new MAF installed and it seems a lot better and it has not thrown any codes yet but it has this odd hesitation.

I would be cruising at 4000 rpm and 50 mph with steady pressure on the throttle pedal and I can suddenly feel a hesitation and small loss of power for a second and then it carries on again with regular power for about 5 seconds(random interval actually) and then has this hesitation again.

Also when punching throttle it revs right up and at over 5k revs it hesitates momentarily.
It does idle fine, and I have reset the computer and done the DME adaptation thing with the ignition on for 1 minute and then off etc.

I did get a few P0507 codes the Idle Air control Valve but since the new MAF install have not gotten that or any other codes.

Seem now like the issue is with higher rev range.
I do have some suction at the oil cap but its not at all strong and the cap come off easy so I don't think its an AOS issue anymore.
I have tried but can not detect any vacuum leaks using TB cleaner spray around hoses.
The gas cap rubber ring does look like its a little cracked so I plan to change that as well since anything vacuum related is the only thing I can think of at this point.
It throws no codes since the new MAF is in place but I suspect a vacuum leak or something else perhaps, although I have not found it yet.

I decided i just dont want to blindly start replacing things and needed to determine how to figure out the problem and fix the actual cause of the problem.

After a bit of online sleuthing I found a series of excellent videos that explains how to use your Diagnostic tools like an ODBII reader or Durametic and determine the nature of your problem without replacing or testing unnecessary components.

I have a Durametric and now know how to use it to figure out what is going on.

Watch these videos and you will be able to save yourself a lot of time and cost and be able to verify repairs done by yourself or the shop.

The first 2 are about learning how the Fuel Trims work and what you can diagnose with them. Fuel Trims Part 1 Fuel Trims Part 2

The 3rd video takes a scenario from start to finish describing the technique to diagnose and verify resolution of an engine problem. Practical engine problem solving Part1 PEPS Part 2

I will do likewise this weekend with my own issues and document how I determined what the problem actually is and verify its resolution.

Wish me luck.

[tonycarreon]

hah! i like this guy.

"if we have these conditions, and you put an oxygen sensor in the car to fix it - you're an idiot."

[tommy583]

I hope you can find the problem. I've had the same problem for over a year and can't figure it out.

[jaykay]

I am experiencing a similar problem but it is intermittent....and random at hwy cruising speeds...no cel.

Soooo it feels almost like bad fuel.....so I will be changing the fuel filter. Not sure if the injector rail pressures are avail via OBD....could log them. Would one see anything
meaningful there ?

Let us know what you find

[NewArt]

Thanks for the link. Really interesting videos! This guy does a great job of explaining a very technical diagnostic tool.

[wawa]

Check the MAF ground. I have a similar problem and last time we find out ground signal was spiking. Sometime signal is good, engine running strait. Sometime signal spike and engine have a thought idle or it hesitate. We have to check three things ( we know MAF is good)
1- MAF plug look in pour shape, might have bad contact
2- ground wire, make sure no short circuit
3-ECU might be going bad

You may want to look at that.

Good luck
Chris

[Jager]

This problem started happening after replacing the AOS? I would double check all the hose connections to the throttle body and the three hoses to the AOS. Those hose connections take some effort to properly seat/lock-in.

[Dragonwind]

This may sound crazy but how's the flow out of your exhaust? Many years ago I had a clogged CAT on a car and it would build too much back pressure. Otherwise you may need to have in indie shop connect their computer to self test various components.

Chris

[jb92563]

Found some free online Porsche workshop manuals
http://workshop-manuals.com/porsche/

I'll be compiling my results and putting together an explanation, but in short for now I found that the RKAT fuel trim was elevated, indicating that the computer is adding extra fuel to compensate for a lean condition, which could be caused by:

- Intake air leak
- low fuel pressure
- exhaust manifold air leak

since the car was running fine prior to my AOS issue it was most likely to be the intake air leak so I loosened and retightened all the intake tube clamps and removed and reattached the upper AOS to intake tube.

the value of RKAT is reduced slightly but the car seems to be running fine, so i'll drive it this week and see if anything further develops.

in the mean time I ordered new fuel cap, spark plugs and fuel filter as these items are due at 70k miles and can only improve the current situation.

more to follow as things progress this week.

[jb92563]

Got a new Fuel filter, Gas cap, Oil cap rubber seal and spark plugs to install.

I still feel its a vacuum leak so I'll probably recheck the new AOS install for leaks.

I have this feeling that when I removed the part of the AOS that sticks into the engine block the rubber seal from the old one did not come out on the spigot so when I installed the new one perhaps it did not seal properly.

I will pull the new one and get the inspection camera in there so I can see if that is the issue, as I'm not sure what the old one had on the spigot for a seal, but its reasonable to assume something like the new one has, which was missing when I pulled the old one.

[jb92563]

After Replacing AOS, and a new MAF there is still a random hesitation on acceleration.

Here are some graphs of
1) FRA RKAT fuel trims, 2) RPM, 3) MAF
Seems to show a need for a bit extra fuel (lean) but nothing drastic and nothing that would indicate a serious vacuum leak. Blue line does spike every now and then, wonder if thats normal.


After replacing plugs and fuel filter, some improvement but not cured.
Showing a need to reduce fuel trim (rich) a bit, perhaps the replaced fuel filter is allowing the extra fuel now.


Another trace after cleaning the throttle body actuator (E-gas) Seems to work same as before. Seems even more balanced than before. Not Likely a vacuum leak
as it would cause the line to be much higher on the + side above +10 (+ indicates Lean condition being compensated for -5 +5 is very normal from what I have read)


After checking fuel pressure, flow rate and smoke test to check for vacuum leaks. No Leaks, pressure and flow to specs.


The hesitation does not necessarily occur where the blue yellow lines spike. Not sure what those are.

I think the next place to look may be fuel pressure.
I'm thinking that the pressure regulator may not be responding properly and causing a hesitation.

Perhaps the thin vacuum line controlling the pressure regulator is leaking.

[Xcellr8]

Just a few words of encouragement to keep going and thanks for the continued postings. I've got a durametric but know very little about how to use it. Learning more and more thanks to posts like yours. Thank You!

[san rensho]

If you are not throwing any codes, and the drive ability problems are at the upper rev range, I would look at the fuel delivery system. Check the fuel pressure and volume delivery of the pump.

[jb92563]

Thanks for the words of encouragement XCELLR8.

Its nice having the tools (Durametric) but if you dont know how to interpret the data your not getting maximum bang out of it.

I bought a couple cigars last evening for my DIY smoke machine for vacuum leak detection. That should be amusing and of course a video will follow with the results.

I'll be getting a Fuel pressure gauge this evening and have read how to measure flow rate....so many liters per minute although I forget the exact number but will document it here when I do the test. (No codes yet)

update- I discoved that Autozone Loans equipment, no charge, so I brought home a nice expensive fuel pressure test gauge for my weekend pressure test...nice surprise.

The car is at 74K miles so replacing a few parts was due anyway so it wasn't unexpected. However the failed AOS precipitated this issue which I find rather odd, so I am expecting to see some vacuum leaks in that area and would make a lot of sense.

Not really expecting a fuel pressure issue but since its easy to check I might as well do it.

I did read something about the Fuel injector rail port where the pressure guage would connect to and wondered if a new washer was necessary when replacing the cap? I read that its a Schroeder valve ?

[JFP in PA]

Originally Posted by jb92563

Thanks for the words of encouragement XCELLR8.

Its nice having the tools (Durametric) but if you dont know how to interpret the data your not getting maximum bang out of it.

I bought a couple cigars last evening for my DIY smoke machine for vacuum leak detection. That should be amusing and of course a video will follow with the results.

I'll be getting a Fuel pressure gauge this evening and have read how to measure flow rate....so many liters per minute although I forget the exact number but will document it here when I do the test. (No codes yet)

The car is at 74K miles so replacing a few parts was due anyway so it wasn't unexpected. However the failed AOS precipitated this issue which I find rather odd, so I am expecting to see some vacuum leaks in that area and would make a lot of sense.

Not really expecting a fuel pressure issue but since its easy to check I might as well do it.

I did read something about the Fuel injector rail port where the pressure guage would connect to and wondered if a new washer was necessary when replacing the cap? I read that its a Schroeder valve ?

It is a metric Schrader valve, and the cap with washer is supposed to be replaced each time, but you can usually get away with taking it off once or twice before it starts to leak. Caps are cheap.

[jb92563]

Fuel pressure is good at idle 45 psi vs spec 47 psi
Beyond idle 57 psi vs 55 psi spec

I measured fuel flow at 3 locations.

just before fuel filter 1650 ml per 30 second s.
at fuel rail schreader valve 350 ml per 30 sec
At fuel return 825ml per 30 sec vs spec 850ml per 30

I also did the smoke testing and found no vacuum leaks except a very minor one at the intake variable length intake tuning flap bearing. Its in the second intake manifold crossover.

Smoke Tester config that worked: Old paint can with a scrap rag soaked in baby oil, crumpled up and lit on fire. Air compressor line into can at 3-5 psi and a line out near the top of the can.
Turn on air compressor, light the rag on fire and close the paint can lid. I also threw the cigar in there for good measure since the cigar only method did not work for me.
It then produce lots of smoke for a couple minutes before you have to relight it. I had to relight a couple times to be fully satisfied that the system was completely smoked through.
I put a plastic bag around the air filter and reinstalled it to effectively seal the intake system. I removed the MAF since you don't want to foul it with smoke residue and I used the MAF opening to introduce the smoke till it exited the oil cap, which I then closed and looked for smoke leaks.
Then I did the test again but this time with the smoke being injected at the oil fill until exiting the MAF opening and then I sealed the MAF opening and checked for leaks.

So no Vacuum leaks is the verdict on that.

The test drive did not exhibit the hesitation this time but seems to run a bit milder with less aggressive more normal acceleration than previous test drives.

Perhaps the computer is relearning and fine tuning the curves?

I wonder what the odds are that the DME computer is having issues or gone bad?
Do those fail often?

[jb92563]

The last Test drive and fiddling introduced new codes P1128, P1130,MAF code, fuel relay. Probably due to my fuel pressure tests as I pulled the fuel pump relay and cranked the engine over to release all residual fuel pressure before doing the fuel flow and pressure tests.

I'll reset all the codes by disconnecting the battery, redo the throttle adaptation and start with a clean slate to see what materializes now that it seems to be running somewhat better.

While I was under the car I did notice some burnt oil residue on the pre Cat O2 sensors on both sides. It looks like it was from the AOS failure as it was not there prior to the event.

My car did pass about 2 quarts of oil during the AOS failure so I'm thinking it could have fouled the O2 sensors and plugged catalytic converters but I am not seeing any codes to indicate that either have failed.

The Durametric trace that I got from the O2 sensors this weekend to see what was going on made little sense as the lines were wandering everywhere.

I was looking for a sine wave on the pre cat sensors and a nearly straight line on the post cat Sensors but got a little bit of everything all mixed together.

It made me think that perhaps I was not getting enough samples per second from the Durametric to see the proper wave form.

I think I need to pull the precat sensors next to inspect them and use my inspection scope to look inside the cats to see if I can see whats going on.

The front side of the exhaust manifold cats look good, no heavy deposits.

[jb92563]

Update.

I replaced the pre CAT O2 sensors and there is no improvement.

I looked inside the CATs with an inspection camera while replacing the O2 sensors and the CATs did not look fouled or damaged in any way.
I also looked at the pistons when replacing the spark plugs and could not make out clearly or understand what I was looking at but it sortof
looked like heavy deposits and the cratered surface of the moon.
Unfortunately the camera would not fit down the spark plug hole so I'm skeptical of what I thought I saw.
Anyone have a reference of what a ~70K mile piston would look like?

I have ruled out:
- Fuel Filter (Replaced)
- Fuel Pressure (Tested OK)
- Fuel Flow (Tested OK)
- Pre CAT O2 sensors (Replaced)
- Air Leaks (Smoke Tested)
- AOS (Replaced)
- Spark Plugs (Replaced)
- Oil cap seal (Replaced)
- Gas cap (Replaced)

I'm still getting the codes 1128, 1130 consistently and sometimes 0507

All the symptoms still point back to the MAF since when I disconnect it the car idles and seems to run normal, although I have not road tested it without MAF.

I returned my new MAF to Pelican and will try another one to see if somehow I got a bad one.

I pray its the MAF because I'm running out of ideas of what it could be.

I considered a bad or intermittent ground or dirty connection but the hesitation happens consistently so a loose connection would be more random I would think.

I had also considered removing the new AOS and reinstalling just in case the port to the engine block is somehow obstructed or to see if the bellows has a tear, although I would have thought that would show up with the smoke test.

[jb92563]

Update

Got the replacement MAF sensor from Pelican but attempting to start the car revealed a dead battery showing 10.9 volts, not enough to start the car.

Once a lead acid battery has died there is usually little chance of it being reliable after that so I simply replaced it without trying to revive it.

With the new MAF in place the same issues are still present. However the P1128 and P1130 codes have not come back.

and the RKAT is up around 4.5 which makes me think perhaps there might be an air leak after all.

[Graph place holder]

Wandering Idle, and a random hesitation.
Only codes P0507, P1571 and P0102 now.

However with the car idling in the driveway for some time while I'm looking at various Durametric parameters being graphed
the idle suddenly goes back to normal and the car seems not to exhibit any symptoms anymore.

I take it for a test drive and everything is back to normal.

I shut down, then start up again and its still fine.

WTF ????

Perhaps a bad ground like Wawa suggested?

Maybe those little random FRA spikes are indicating a wiring problem.

I hate these kind of problems, a pain to debug, and if you don't find the issue you can never really trust it to be reliable.

I'll have to start wiggling wires to see if that might be the issue.

[Reebuck1]

Maybe the DME hit it's learning time set and brought the car back to the original programing. Not sure of the time frames for each or all the adjustments. Have had problems that were corrected during an extended test drive as the computer learns and adjusts the cars program.