'01 L67 - Hesitation at partial throttle up to WOT

[digitaloutsider]

X-post from the FB group. Sorry!
 

I am so completely out of ideas so I figured I'd give it a shot asking here. Problem with my '01 GS. Weird stumbling issue at half throttle up to WOT.

 

It's done this since the day I bought the car, but it seems to be getting incrementally worse. At first, it was just at WOT. There would be a mild stumbling/hesitation. It truly felt like a spark problem, it feels like when I've previously had a weak coil or fueling problems. Anyway, I also noticed that the tach would jump around, bouncing from 0 and back up. I replaced the CKPS and it fixed the tach, but still hesitated.

 

Figured it's 116k so I should do the plugs and wires. OEM-spec ACDelco plugs and ACDelco wires. Still hesitated. Thought maybe it was the MAF. Unplugged the MAF and went for a drive, same problem. I got a full set of coils and ICM from an '04 Impala for really cheap, so I threw those on just for shits and giggles, same problem.

 

I did remember that my '98 had a VERY similar issue at one point in the past and it was due to a faulty O2 sensor. Figured given the mileage it could use a new one anyway, so I threw a new Denso on. Same problem. Put a new fuel filter on as well since that's long overdue.

 

I tried to rent a fuel pressure tester, but AutoZone gave me two broken ones in a row, and given the amount of gas that went pouring all over the place I really doubt this to be a fueling problem. I have scanned the car and the readings from the TPS and MAP both look to be fine (not jumping all around as if they were faulty/had bad wiring). No KR except for a blip here and there, so the knock detection seems to be fine. Reading from the IAC is correct.

 

This also doesn't happen ALL the time, sometimes it barely hesitates, especially when going WOT at interstate speeds. The absolute worst time is when you're doing a 3-1 downshift at like 35 MPH or so. It'll hesitate as it shifts down, then surge and hesitate some more after it shifts.

 

tl;dr: stumbling most anywhere over 50% throttle. Worst at WOT. Doesn't happen all the time. Car has new plugs, wires, ICM, coils, CKPS, fuel filter, and O2 sensor. ANY ideas at all? Because I'm seriously out of any good ones.

[94 olds vert]

My l67 had this problem.I tried unplugging my MAF,but I still had the problem too. It turned out to be the MAF, and it never set a code. I cleaned it, the problem came back.I put a new MAF in and I haven't had a problem. I used a scan tool to check my MAF.

Edited April 17, 2016 by 94 olds vert

[Padgett]

This is where you really need a live scanner (under $40 for OBD-II which you have). That said the tach jumping around is the clue. No codes generally means secondary ignition but the tach is driven by the ICM and that isn't bad.

 

OK with a L67 you got a special ICM/coils that did not come with the L27. What engine did the Impala have ?

 

Finally when all else fails try pulling the control wires from the alternator and go for a short drive. If runs better you either have a failing alternator or a bad ground wire from the battery to the chassis.

[digitaloutsider]

I do have a scanner(s) and I've scanned every datapoint I can think of: MAP, MAF, O2, etc.  Nothing seems out of spec.

 

The tach issue is 100% indicative of a failing CKPS, and changing it fixed that problem. I even did a CASE learn just to be sure nothing is up with it. The coils/ICM are off of a standard poverty-spec 3800 Impala, and are shared with most every 90 degree in existence. 

 

I did pull the SC belt and the problem is either gone or completely un-noticeable.  Which leads to believe everything ignition is fine, as are most of the sensors.  That doesn't eliminate a weak fuel pump, but for fucks sake I cannot find a not-broken rental fuel pressure tester at any autozone around here.  I've been through 6 of their units so far and all are broken in various ways.  An interesting point was that I was actually seeing boost on on the DIC when I was railing on the car tonight. 1.5-2psi.. with no belt. I can't possibly imagine that it would ever create boost just from the rotors spinning with no belt.  Maybe a bad MAP?

[Padgett]

After a long series of bad/leaking fuel pressure testers, I bought one of these. It works.

 

Could be getting a bit of ram effect. GM puts the air cleaner snout in the pressure area in front of the radiator.

 

Impala uses D555 coils, L67 D576. Are a bit hotter.

 

Hook the S/C belt back on and pull the alternator control plug. The alternator has a lot of noise in it, the rectifier bridge has no damping & relies on the battery to be a very large inductive filter. A bad ground can put that high frequency noise in the computer and make things go wonkey. Have seen a bad ground cause gauges to jump with the turn signal, surges of up to 200v, and other nasty things. Did I mention I spent five years at Delco Remy playing with a lot of things ?

 

ps I assume you meant the Crank Position Sensor. could you see any sign that the rings were hitting the magnets ?

Edited April 18, 2016 by Padgett

[mfewtrail]

I wouldn't condemn the map just yet. Under heavy throttle without the sc belt I would not be surprised to see that a small amount of boost can actually be made from the air entering the engine turning the rotors. Someone with a sc'd 3800 might have made this observation before when running with no belt. I seem to remember a posting about this long ago, but I might be mistaken. Do you have any way to record the data from your scan tool while the problem occurs?

[digitaloutsider]

I unplugged the MAP and had the same issue so the MAP is okay I guess.  I'll try to get a scan later tonight but at this point everything seems to be fueling related. 

 

Also that fuel pressure gauge above is the exact one that AZ rents out.  There is no way in piss I'm paying and money for that.

[Psych0matt]

Order a new map anyway? They're like $10. Couldn't hurt. Sorry I'm not very helpful

[digitaloutsider]

Well I'm happy to report I fixed the problem. Yesterday on a gut instinct I picked up a set of AL104s. Today on lunch I ripped the brand new (put them in maybe 800mi ago) stock ACDelcos out and threw the 104s in.  No hesitation, no surging, pulls like a freight train.  I don't know if likes the colder plugs better or what, but I don't give a shit at this point.  I'm just thrilled it runs right for once.