alright, here it goes...

[baby6]

Lets start off the right way. The car is an 86 GN, freshly rebuilt 109 .020 over, 206/206 flat tappet cam, 60lb injectors, lt1 maf with translator, racetronix fp and hangar with hotwire kit...the basics. Fp set to 45 line off.

It always has problems, but there are two I CANNOT figure out. First one- car does not like to idle. Vacuum will bounce, and sometimes the car will die. Blms are good 122-128 at idle, maf is known to be good. When it starts to hesitate (almost like multiple misfires at once) I can watch the 02s drop to almost noything on the scanmaster, and when it gets over its moment the 02s will come back up and continue looping. Tps is set at .40 and iac is around 20 when hot. On occasion it will idle fine, but it seems now to happen less and less.

What I have tried - readjusting cam sensor, new IAC, new plugs and wires, another ignition module and coil pack, computer.....and it seems to be getting worse. Tps is good. Have also tries another maf and fp regulator.

Secondly, the car had no misfires at WOT before rebuild. Now if I go WOT, I notice that my boost fluxuates, and the car starts to sputter really bad. Seems to happen more warm than cold. Again, tried another coil and module (used ac delco I believe) and same exact thing. New plugs and wires as well. Computer is good, cam sensor seems to be working fine. Definitely getting fuel.

Please help me. I am desperate for help. This thing gives me a huuuuge headache, usually I can figure them out but these two problems have me STUMPED. I was thinking maybe the crank sensor could be an issue at WOT, but the idling problem has been happening for years.

 

[Street Lethal]

Lets start off the right way. The car is an 86 GN, freshly rebuilt 109 .020 over, 206/206 flat tappet cam, 60lb injectors, lt1 maf with translator, racetronix fp and hangar with hotwire kit...the basics. Fp set to 45 line off.

What is the fuel pressure when it starts to act up...?

Have you double checked the grounds in the ECM harness...?

 

[baby6]

Fuel pressure is right where its supposed to be, it bounces a little when the vacuum starts to bounce but no severe drop to make it stumble. How many grounds are there for the ecm and where are they?

 

[HouTX87]

I'd run a compression test. I've read a lot of stories about new flat tappets getting wiped out if they aren't broken in properly. How did you break in the cam?

 

[TLIMITED]

I had a couple of cars over the years with somewhat same problems.It was the cam sensor on both of them.Also check vacuum hose at MAP sensor.

 

[baby6]

Vacuum line at map is good. Car still stumbles funny at idle with cam sensor disconnected. Ill run a compression test soon, but I doubt its that, feels electrical. Cam was broken in at 2000rpms for 20 minutes, and had zync additive in oil for 5000 miles

 

[Street Lethal]

Fuel pressure is right where its supposed to be, it bounces a little when the vacuum starts to bounce but no severe drop to make it stumble. How many grounds are there for the ecm and where are they?

Follow the ECM harness as it goes past the engine block to find the grounds, make sure they are all tight and secure. Fuel pressure shouldn't fall when vacuum starts to fall, it should remain constant. Being that the fuel pressure gauge does flicker though in conjunction with vacuum, and if you feel it is in fact electrical, then check the charging system; the alternator itself, as well as the connections behind it. If the alternator is bad, or if the connection to the battery leading to the alternator is loose, you will have similar symptoms with no ECM error codes being thrown...

Did you check your O2 sensor's voltage yet...?

 

[dvernst]

Follow the ECM harness as it goes past the engine block to find the grounds, make sure they are all tight and secure. Fuel pressure shouldn't fall when vacuum starts to fall, it should remain constant. Being that the fuel pressure gauge does flicker though in conjunction with vacuum, and if you feel it is in fact electrical, then check the charging system; the alternator itself, as well as the connections behind it. If the alternator is bad, or if the connection to the battery leading to the alternator is loose, you will have similar symptoms with no ECM error codes being thrown...

Did you check your O2 sensor's voltage yet...?

If it loses vacuum fuel pressure will rise. Just like taking the vacuum line off the regulator?

 

[Street Lethal]

If it loses vacuum fuel pressure will rise. Just like taking the vacuum line off the regulator?

No no, that isn't what I was saying. He is claiming that he feels it is an electrical problem, and if the vacuum gauge is going low followed by the fuel pressure gauge going low too at the same time, it might be the charging system because the fuel pump should not be losing pressure unless it loses current. The injectors might not be getting full voltage because of a bad ground, or a bad alternator/alternator connection, with the ECM constantly trying to correct causing his hesitation. In normal conditions, fuel pressure rises when vacuum decreases...

 

[dvernst]

No no, that isn't what I was saying. He is claiming that he feels it is an electrical problem, and if the vacuum gauge is going low followed by the fuel pressure gauge going low too at the same time, it might be the charging system because the fuel pump should not be losing pressure unless it loses current. The injectors might not be getting full voltage because of a bad ground, or a bad alternator/alternator connection, with the ECM constantly trying to correct causing his hesitation. In normal conditions, fuel pressure rises when vacuum decreases...

OK got ya!

 

[baby6]

I apologize, I meant to say fp comes UP not down when the car loses vacuum.
How do I check voltage to 02 sensor? Just volt it out? Ive also put a new 02 in, definitely looped better but no difference in idle quality....its almost like the car is hunting, but my blms are usually great at idle in closed loop.

 

[baby6]

Oh yeab, alternator and all connections are good. Batteey as well.

 

[Street Lethal]

Ive also put a new 02 in, definitely looped better but no difference in idle quality....its almost like the car is hunting, but my blms are usually great at idle in closed loop.

Although the SES light may be off, are there any codes being stored possibly?

How do I check voltage to 02 sensor? Just volt it out?

Assuming your running a one wire sensor, use a high impedance digital volt meter, connect the negative lead from the meter to the O2 sensor's metal enclosure, and the positive lead to the connector wire. Using a propane torch, heat up the sensor, and watch for a reading above 650mV. It will vary as you move the propane torch on and off of it...

 

[baby6]

No codes stored. Ill look into testing the 02 sensor in the near future. I can nearly guarantee thats not the problem but I can at least verify!

 

[YoungGNowner]

You said it idles better in closed loop, but have you ever checked out your Long Term, and Short Term fuel trims(If you can).. i would also try a new MAP sensor. Also what your Fuel ratios looking like at WOT and idle?

 

[Street Lethal]

i would also try a new MAP sensor...

The MAP sensor isn't controlled by the ECM, it is only there for the boost gauge in the cluster...

 

[baby6]

I am so tempted to sell/trade this thing sometimes...but then I think, "how would my life be without it?" I am now 21, and have had the car since age 14 (backyard find, my dad bought it for me). And ever since that day ive been married to it and fixing it/enjoying it.. I desperately want to figure out whats going on. Note- I whacked a telephonw pole in the passenger rear quarter about 4 years ago. If I recall, it did not stumble at idle prior to accident. I have checked the wiring in back all the way to the ground distribution point in the frame, everything looked perfect. Is there any significant wiring on the passenger rear of vehicle? Please keep the input going!!!

 

[1badbuford]

Did you check lifter preload when you assembled the motor? And did you ever do a compression test? Is the egr still hooked up and functioning or is it blocked off?

 

[baby6]

Did not check lifter preload. Compression good, egr is blocked off

 

[1badbuford]

Is it backfiring at all?

May need to see if is pumping up the lifters and hanging the valves open on exhaust just a little bit when its running, Would be rare but could happen depending on if heads were milled down and they used stock length pushrods . Is it a stock rocker shaft with non adj rockers ? If so, and you can't find anything else, you could pull drivers valve cover, roll any of the pistons to tdc by watching the exhaust valve close and make sure intake doesn't start to open, then verify that piston is at tdc by pulling plug and use something to stock down in plug hole that is clean... Then loosen the shaft and when re tightening , start spinning one of the pushrods on the cyl that is at tdc, then once you barely feel friction , slack is out . From that Point try and see how many turns you get on shaft bolt before the pushrod on that hole stop going down, or get a adj, pushrod and check it to see exactly what length you need and compare with what you have.