348 Engine repair thread

When I say rough idle I mean Loping like a funny car,, BA DA DUM, BA DA DUM, with rpm dropping almost to stall the back up and down. You can hear the banks fighting almost.

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Mine does it when cold, really bad, when warm as i. Fans coming on it's fine. Usually 02 affect when it's warm not when cold? I'll reset the intakes and test the iac's I already verified they get 11.8v. I'll also spray starter fluid when cold to check for vacum leaks. Waiting on connectors to make a harness to reserve the tps sensors. I'll report back.

Is there an iac synch procedure I wonder?

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Thanks all its helping. Right one is pretty much all the way in, the other is like 1/2 turn out. Both banks are just in the red below 18 inches. I suspect the IAC's also, because according to the manual they are open and close gradually when as the car gets warmed up to operating temp. Car is fine at operating temp. Getting the IAC into range has me wondering too. No mention on a reset/resynch procedure that I could find like modern cars.

I will pull them and bench test them with a 9V battery. If good I will ensure they are open fully before installing them. I will also make a 2 pin version of the bypass harness and verify voltage when installed, perhaps the voltage pin connector in the meddle is too spread out and is not contacting. Either way will verify. the plug is definitely getting 11.8v on both sides. Whether that's getting to the IAC is a different condition.

Found them here.
https://www.corsa-technic.com/item.php?item_id=1040
https://www.corsa-technic.com/item.php?item_id=648
http://www.cycleterminal.com/junior-power-timer.html


From the manual page C19.
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Its well below 50 here all right. Cleaned grime the bleed screws as well, nice and smooth. "Seem" to recall this happened every winter actually. I assumed it was bad gas. This idle issue though is like really bad when cold like banks loping almost to a stall then back up but never to idle speed until warmed up. Almost like banks are fighting when cold.

 

Those were all replaced last year, like 300 miles on them, working great before the pully failed. Not replaced when I rebuilt, sorry for the ambuguity.

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Agreed... my idle screws are a separate issue, that balance is done when it at operating temp yes. More of another thing indicative of something with them being all the way in almost. Could be plenum leak too causing both, will check that at same time.

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Not really sort of but it bogs down, not responsive like you would think.

 

Ahh Ok thanks! I think the build was successful, this issue happens every winter, it started when I adjusted things like the forum says, MAF's in particular. The TPS I will reset correctly shortly. That leaves MAF's.

But with a rebuild I am sure what ever they were are not valid anyhow. Need to figure out how to set the CO2 screws perhaps. After I reset the butterflies and verity the IAC's if needed at that point. Maybe start with cat temps to see where they are. But not sure that helps as when hot its in closed loop, before the cats are hot enough to measure, the condition is cold or before the car is at operating temps. Can't really measure cold cats to see what the MAF is sending to the car.

I knew a 348 owned by one guy for over 20 years it is unmolested, and he knows the guy he bought it from, still has the blue caps covering the CO2 screws, perhaps I will measure his MAFs to see what the factory set them too, I bet anything they are not 383, just for a data point.

Like John says feels sort of like a MAF, but these are 2 brand new MAF's and my old Maf's did this too, I thought the new MAFs fixed it but it was just likely due to the warmer weather when I put them on.

Oh when the car is operating temp, if I shut it off and restart it, the idle is fine. If I let it cool down to where I can touch coolant tempos, it starts again. The blue temp sensors are also new.

No check engine lights and yes both CEL lights come on when ignition it turned on. So they work. Actually that whole entire panel is new as of 2 years ago.

 

Right yes adjusts the signal for the ECU. And yes clockwise, higher is richer. They are currently 383. I can also put a harness on both sides of the TPC and verify the open at the same time after I set them to .05mm. Waiting on new feelers and plugs currently. We'll see after I reset everything and move on from that point. I know before I touched the MAF's one was like 370 and one was like 410 (memory is fuzzy there) but but the caps were off so one had to assume someone touched them.

 

According to an old Marco Bussadori post "The ohm settings don't matter on the MAF unless it runs open loop. You will run Open loop at cold startup until the O2's get warmed up to operating temp. After that unless the O2's drop off your ECU will not read the MAF CO screw."

 

Not sure if this helps but I just tested with 9v battery, and it snaps open quick and fully closes quick when from 9v battery + applied to center pin and - from is applied to either end pin. With no power there is about a 2mm open slit as mentioned, I assumed spring loaded but not sure how its open 2mm since it closes fully shut when power is applied so it cant be a stop.

No clue how I test for - to end pins when it's in the car, sounds like oscilloscope is needed for anything other than center pin 11.8 volt check.

 

Been reading on O2 sensors, if 02 sensor turns oxygen content into a signal (mv) then when normal condition there is also a signal. One can find that value, 450mv or so with higher mv being rich and lower being lean according to 02 docs I found, then one should be able to set maf open loop setting from unplugged 02 sensor wire at operating temp, or get it into the ball park, I think... I may get to that but I suspect that's how indy shops do it...

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