new guy need help tuning

[gnn'ak]

Hi all,
I have been chasing this problem in my 86 GN for a few weeks now and I cannot figure it out. The problem is that it has a random stumble all through out the RPM range, like one cylinder is not firing/combusting correctly but its not consistent. Also when I first start it up it idles smooth as glass and when it warms up a bit after about 45 seconds to a minuet the stumble/shake comes and stays. It is a brand new engine that I have put about 25 miles on. I have set the cam sensor twice. I have also reset the IAC and adj. the TPS at least a dozen times. When it first goes into closed loop the IAC is about 25, but it randomly drops to 0 somtimes and stays there at idle. The current TPS reading is .044. I had a mechanic buddy of mine graph the injectors on his scan tool and he said they looked fine although the #1 injector is alot louder than the rest. I also put new AC Delco plugs in and new plug wires. The MAF sensor passes the tapping test and airflow is with in spec. I put a new fuel filter in, new O2 sensor, and a bunch of things I cant remember right now. Im tempted to replace my coilpack but I dont want to keep throwing money at the problem. Sorry for the long post. Any help would be appreciated. TIA

 

[Blown&Injected]

Welcome to the board!

Looks like you are doing a goo job of tracking it down even though no good results.

Do you have a vacuum gauge?

I have had to go thru 2 sets of plug wires before grtting my engine to run right - sometimes even new stuff is bad.

 

[XLR8]

After the car is running unplug the cam sensor and see if the miss goes away. I had a recent similar problem (but only missed at low RPM) and it was the cam sensor - when I unplugged it, it ran smooth as silk.

 

[gnn'ak]

Thanks for the replys,
I will borrow a vacuum gauge then and check and post back the readings, also I will unplug the cam sensor while its running and see what happens. Thanks again

 

[Heath]

check all your grounds. :smile:

 

[gnn'ak]

I went today and checked the vacuum and it read 13 +-1 it kinda jumped around with the idle. I also unplugged the cam sensor when it was running and there was no change. Thanks again for the help.

 

[alvin87gn]

ive put ac delco plugs in a car that ran fine and it made it run like crap. we quit usin them at work because they have quality control problems. id run the ngk's or almost anything but ac delco. feel yer plugs and see if the porcelein is loose at all, usually some of them like to twist. not that i'm guaranteeing this is the problem but ive fought lots o them ac devil plugs

 

[gnn'ak]

Yea I think that I will give the NGK's a try tommorow. Also I checked all the grounds and everything looks fine. There is a little red wire coming off of the positive batt. cable that goes to a fuseable link that is about half ripped. When I unplug the wire the car shuts off, so I guess its still making connection?

 

[BlackBandit]

That's your ECM power wire. Make sure the connection is good on that because it can give you REALLY WIERD PROBLEMS. Not that I know from experiance or anything. :redface: Probably not you current problem though. If that conection was intermitant your scanmaster would probably be resetting every so often.

 

[VadersV6]

You can tell when the computer switches to closed loop after startup, by watching the blinking led on the scanmaster. Does it happen right when you hit closed loop?
Check the drivers side header where the 2 rear pipes are welded together. Look very closely for cracks.
Once you hit closed loop, how active are your O2 crosscounts, what does AF say on the scanmaster and what are your BLM's in closed loop?
You may have a pinched/bad wire or connector going to the IAC, or a bad IAC itself. It shouldnt be dropping to zero. An intermittent connection there could wreak havoc on everything.
I had a bad MAF and tapping on it didnt prove anything. The AF reading at idle and cruise will tell you if its in decent shape. Is it a stock MAF and do you have a translator if its not?
There is nothing wrong with AC plugs as long as you dont crack them or gap them wrong.
Do an ohm check on all your plug wires (I had 1 bad wire causing stumbling...and they were new), and do an ohm check on all 3 coils.
I would tackle this IAC issue before touching anything else.

 

[gnn'ak]

Thank you all for your comments, I went today and did a little more trouble shooting

Does it happen right when you hit closed loop?

The random stumble/shake happens in open loop too, The only time it stops is like the first 45 seconds after startup and it pretty much stops if I rev it up to around 3200 RPM.

Check the drivers side header where the 2 rear pipes are welded together. Look very closely for cracks.

My old ones were cracked so when I put the new engine in I put on a new set of Poston headers.

Once you hit closed loop, how active are your O2 crosscounts, what does AF say on the scanmaster and what are your BLM's in closed loop?

O2 cross counts= 7-20
BLM=131

The AF reading at idle and cruise will tell you if its in decent shape. Is it a stock MAF and do you have a translator if its not?

Its the stock MAF and the Air Flow at idle is 5-6g/s

Tonight Ill do an ohm check on all the wires.

and do an ohm check on all 3 coils.

the readings from the coils were 11.49, 11.39, 11.47. I also put on a new coil and module today but unfourtunatly it didnt fix it The frustrating thing is that I bought the car with a blown engine so I have no idea about how it ran before I bought it. And the previous owner didnt know anything about it and said that it ran ok before it threw a rod. Thanks again for all the help

 

[gnn'ak]

Do an ohm check on all your plug wires (I had 1 bad wire causing stumbling...and they were new)

I did the ohm check on all the wires tonight.
Plug wire #1=3.472 ohms per foot
#2=3.308
#3=3.480
#4=3.270
#5=3.310
#6=3.429
I think that I read somewhere that they were all supposed to be with in 10% of eachother so I guess by that they look ok. Thanks

 

[VadersV6]

Your wires are fine. The response to the crosscount activity doesnt really mean anything. It should cycle from 0 to 255 and start over again within 90 seconds. With my Bosch O2 sensor, it will cycle in about 10-15 seconds vs. 60-90 seconds for the new Denso I used to have on it.
The IAC sounds like the main issue here. It should not go to zero. Ever. Either there is a bad wire or connection, or its way out of adjustment or bad.
There is an article on gnttype.org on how to set the IAC. Its the best possible way of adjusting it. Basically, you warm the engine up and then shut it off once it reaches operating temp. Then turn the ignition on and jump 2 pins on the ALDL connector (gnttype tells you which ones) in order to send voltage to the IAC motor, which pushes the IAC plunger all the way forward to where it completely seals the air bypass hole. Then you shut the ignition off so it stays there. Then you pull the jumper out of the ALDL, and then unplug the IAC. Then you turn the idle speed adjustment screw in a few turns in order to open up the throttle blades a bit to let in the air that the IAC will not be letting in (since its sealed and unplugged). Gnttype doesnt tell you to do this..I dont think..but it helps alot. Then you turn the ignition back on and start the motor. Slowly turn the idle speed screw out until the engine just barely dies. Now you shut the ignition off and plug the IAC connector back in, and then adjust your TPS sensor. You are now ready to rock. This procedure is pretty much the same on all EFI cars. Now if you end up running into the same issue of zero voltage, you have a wire problem, connector problem, or the IAC stepper motor is bad.