Still chasing that weird idle pulsing.

[Joewwy]

gm 350 truck crate motor, 5k miles.

So far went to atomic efi, efi is controlling timing, new plugs with maybe 500 miles on them, new wires, replaced pcv valve, added a oil seperator to pcv line to keep oil out of intake.


Ok so did a compression test on all 8 cyl each were at 140 and very slowly releasing the pressure. No difference between any cylinders. Look at these 4 plugs and tell me what you think. Those are 2 4 6 and 8 2 on bottom and 8 on top.



6 and 8



2 and 4

[fauxrs]

Are you running vacuum advance?

If you are running manifold vacuum to your vacuum advance, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2 in. Hg below (numerically less
than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling.

This will cause erratic timing with associated unstable idle rpm.

[Joewwy]

only brake booster and pcv are using a vacuum.

[darbysan]

Your Truck Crate engine- does it by chance have Vortec Heads? If so, the plugs are wrong.

[Joewwy]

no vortec heads. It said truck when i bought it, i'm pretty sure but it is not a vortec.

[Joewwy]

Ill take that the lack of concern about what the spark plugs look like that they are normal.

[Joewwy]

Tried propane to search for vacuum leaks, not getting any difference.

[brddg]

How erratic is the idle? that system uses a wide band o2 right? Where did you plant the o2 sensor?

[Joewwy]

Originally Posted by brddg

How erratic is the idle? that system uses a wide band o2 right? Where did you plant the o2 sensor?

100 rpm maybe a little more. It does have a 02 sensor it is just past where the header tubes connect just like they show on their video car.

[jlcustomz]

The plugs do look a hint like they might be a little on the cold side temperature range,although not your issue. color looks close enough to a light tan.
This msd system is still relatively new to us, have you tried their tech support yet? This may be your best bet. Sounds as if the idle air control is doing to much searching. Is there an idle stop screw you can adjust? This may manually alter the way the computer reads & handles idle speed.

Sometimes issues aren't something of our fault, although the still are our problem.
Also do you have a good enough alternator{ at low rpm).Good clean grounds & wiring connections? Other things like this can effect efi.

[Joewwy]

The problem has been around since before the efi system, much worse but still there. I'll try msd when I get a chance. Alternator is stock to the elco and a reman not a year old yet. No lose wires all grounds look fine and don't budge.

[brddg]

How big is the cam??? I ask because sometimes a lot of overlap will always have low rpm idle issues efi or not. 1000 sounds kind of high for idle. I rememer back in carb only days guys would uncover the idle transition slot and have idle and plug fouling problems.

[Joewwy]

Stock cam, 112 seperation. I have it idling at 750 range. It has a small back firing in exhaust more like thuds than pings with an open throttle. If engine is cold it would start fine if its warm it would try to die(efi stops that now(still drops down though)). Ignition has been replaced a couple of times. The only time it got better was cold outside roughly 40 I think and I just replaced the pcv valve. Later that day though it was back to the same as before.

[Jakes72]

we had the same problem with my brothers truck when we put fuel injection on it and it ended up that we didnt have enough ground straps on it

[Joewwy]

Originally Posted by Jakes72

we had the same problem with my brothers truck when we put fuel injection on it and it ended up that we didnt have enough ground straps on it

That would awesome if its that simple. How did you discover that out?