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My "new to me" car has been experiencing rough idle since I got her earlier this year.
Have replaced the HT leads. Distributor caps look like new. Spark plugs were changed by the PO only a few thousand miles ago.
Cleaned the ISV. Replaced the MAF.
Its still there.
You can hear it in the attached video.
The check engine light never comes on. Car passed Smog back in February once I'd completed the drive cycles correctly.
Bought a used Durametric cable on ebay this week.
There're sporadic glitches in most of the readings - either O2 sensor, MAF airflow, etc....
Is this normal? Related?
Wondering what could be causing this... anyone have any thoughts??
Hi Foxman - thanks for that suggestion
I checked that too! Engine runs on each of the two dizzy halves, so I infer that the belt is not broken.
It seems most of the signals have glitches in them. I started wondering about cause and effect, but now I'm wondering if there's some electrical earth problem...
Hi Foxman - thanks for that suggestion
I checked that too! Engine runs on each of the two dizzy halves, so I infer that the belt is not broken.
It seems most of the signals have glitches in them. I started wondering about cause and effect, but now I'm wondering if there's some electrical earth problem...
Youve covered most of the bases. Agree it could be a loose ground, or perhaps related to the alternator. Is the fan rubbing the housing at all?
Sporadic glitches in all readings is odd. Maybe start by cleaning your grounds. Clean the battery ground, then find the ground points in the car and go after them:
Foxman - Fan isn't rubbing.
EckFe1 - thats an interesting angle - can I run the engine with the knock sensors disconnected as a test??
Abiazis - fuel filter is something I was thinking of too. Its on my list for the Winter maintenance list.
For ground wire issues - the reported battery voltage is steady as a rock at 14.3V (Engine running)... so that makes me wonder if ground isn't the issue...I'm slightly reluctant to disconnect the battery, as that means I've got the dang pesky Smog emissions drive cycle to do all over again... (sigh - I'm in CA).
Thanks everyone for the ideas... I'll keep digging!
Haven't used my Durametric for a while so I'm not sure if you can select the knock sensor telemetry with it.
My thinking is, if there's an implausible signal or loose connection, it might alter ignition timing.
I believe the wrong belts can cause similar issues.
Hi Ardvark,
A used Durametric cable on eBay, could the cable or its connections be bad, everything used purchased on eBay is suspect. Did you download the latest software from Durametric's website? Also, consider cleaning the OBD plug and socket with a spray electronic cleaner. Verify your cable is the later version. The earlier version had a box along the cable in addition to the end connectors and has compatibility issues.
Do you hear or feel the idle valve humming or buzzing, indicating that it is working? Also, the wiring harness plug that fits it has a rubber seal in the bottom of it that can dislodge from its correct position interfering with the plug fully seating and the idle valve working.
Check the actual data from the MAF sensor as it may have been bad out of the box as new.
The only time I ever had symptoms as you have was with my Motronic 1986 Carrera in extreme cold weather it would cycle up and down at idle until it warmed up. This is why I say check the air flow sensor.
When you say the caps look like new do you refer to the absence of carbon trails around the interior of the caps and free of carbon on the tip of the rotor?
Andy
Last edited by pp000830; Sep 25, 2023 at 06:23 PM.
Do the glitches on the scope correlate with rough running?
the fact you get 14.3v on the battery terminals says the ground from battery to chassis and starter/alternator to chassis are good. But theres the hard to get to engine ground, the one that gets all the oil and road crud on it.
im with the crowd, its an electrical intermittent in the engine harness, most likely ground but worth checking all the engine wiring connector behind the rear blower. Disconnect them, squirt with electrical contact cleaner spray and reconnect, repeat a few times to wipe the contacts well.
I cant remember where the engine ground wire is located. Im sure someone here will oblige
@Andy - I bought the Durametric as a "confirmation" test for the Bluetooth OBD tool I was using with my phone, which was also showing these sporadic glitches. I was kinda hoping it was all a hoax (even though its clear from the exhaust behavior that there's something not right). Durametric cable is a later design cable, and the SW is the current release from their website.
ISV - I can't feel any vibration above that of the engine, but if I disconnect it, the idle speed rises slightly.
MAF - this is a brand new MAF, and behaves the same as the original (which I'd also previously cleaned with MAF spray to no avail).
@IanM - hard to tell on correlation - need to play on the Durametric a bit more.
On my phone ODB app, I note that the RPM spikes are *positive* - up to 11k rpm or more whilst otherwise at idle(need to find time to check with the durametric tool). Wondering if this is significant and the cause of everything else going to zero)?
@Gbos1 - forgot - I already changed the DME. Vaccum lines are good and hold pressure for many days.
@Jay - if you put your hand in front of the exhaust, you can feel the irregular puffs of gas from the engine.
If anyone is going to the Fahrspaß gathering tomorrow in Monterey, I'd appreciate some in-person feedback!
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Rough running
