Timing advance and distributor rotor
- Thread starter Rubberup
- Start date
AaronPaul
TJ Enthusiast
Timing at idle I would expect to jump around but closer to 10-12 degrees like people have said. If it is dead steady at 16 then the ecu could still be in open loop for cold start warm up or something. However, on higher compression engines, this can get bumped up to 15-20 degrees without issues. in normal cruising under light throttle it might not be unusual to see 30-40 degrees of timing. This is because leaner mixtures typical under high vacuum conditions need more advance for the mixture to light off. This is why old cars with points ignition and mechanical distributors used vacuum advance for better mileage and light throttle drivability.
Under full throttle, high rpm, like above 3,000, you should see the max advance, which typically is between 32 and 36 degrees of advance. as rpms increase, the timing needs to be increased in order to have the mixture ignite at the correct time for the power stroke and before the exhaust valve is opened.
Under moderate throttle (enough that there is no vacuum produced in the manifold) and lower rpm, the computer is going to adjust the timing based on airflow/manifold pressure, throttle position, and some other factors, but i would expect to see something between the idle setting and 20-25 degrees, up to whatever the max WOT timing above.
These are all general numbers, not specific to the jeep engines, but they are generally typical.
Were the readings at idle with the engine fully up to temp? Engines get a ton of extra fuel on cold startup and during warm up for various reasons (some situations a cold start can require 200% of normal hot idle fueling to start and run), so just want to make sure that is not a factor that might contribute to higher idle timing.
Under full throttle, high rpm, like above 3,000, you should see the max advance, which typically is between 32 and 36 degrees of advance. as rpms increase, the timing needs to be increased in order to have the mixture ignite at the correct time for the power stroke and before the exhaust valve is opened.
Under moderate throttle (enough that there is no vacuum produced in the manifold) and lower rpm, the computer is going to adjust the timing based on airflow/manifold pressure, throttle position, and some other factors, but i would expect to see something between the idle setting and 20-25 degrees, up to whatever the max WOT timing above.
These are all general numbers, not specific to the jeep engines, but they are generally typical.
Were the readings at idle with the engine fully up to temp? Engines get a ton of extra fuel on cold startup and during warm up for various reasons (some situations a cold start can require 200% of normal hot idle fueling to start and run), so just want to make sure that is not a factor that might contribute to higher idle timing.
AaronPaul
TJ Enthusiast
"These are all general numbers, not specific to the jeep engines, but they are generally typical." That is exactly what I said. I am happy to see someone else agrees lol.
lol yeah I haven't messed with jeeps too much, but i've played with engines just about my whole life.
AaronPaul
TJ Enthusiast
AaronPaul, Thank you!!
That helps a lot.
I was getting 16 BTDC at idle which was off as I suspected.
I replaced a few sensors, tested each and some were out a tiny bit (MAP, IAT, coolant temp, TPS, CPS). Which seemed to help starting and idling a tiny amount.
Then, I replaced the distributor (about $10 more than the cost of the cam position sensor alone). AND now idle is 12.5-14 BTDC and smoother! I found 3 things off.
The old distributor had the "ears" ground off it (presumably for adjustment), even though it seemed to be straight on the bolt.
It had no distributor gasket, not a big deal, but nice to have.
The distributor seemed to be a tooth off (more below on this).
When I put the new distributor in (with oil pump at 10 o'clock), with the pin in the alignment hole, the "fork" would not line up with the bolt. It was always just a tad past it. I found one video on line where the person removed the lower dist. gear (roll pin first) and rotated it 180 degrees, which moved it about 1/2 a tooth. I could have done this. But, then I remembered that the rotor was not pointed directly at the number one cap terminal before (it was just after it, but still near enough to fire across the ears). So, I carefully aligned everything again (except put the oil pump at 9:30) to insert the distributor and removed the alignment pin, allowed the rotor to turn counterclockwise a tiny amount and it dropped into place.
As an added note, and thank you, your reply was especially helpful as the manual does not list any idle or running timing measurements. And, when troubleshooting, a code reader or timing light will output this info.
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