Hesitation below 2000 rpm's

[MN6WS6]

Hey guys. I've been having an issue with the car for a while now, and it's gotten to the point where I just don't know what to do anymore. "A while" is defined as "at least two years" if you're wondering. I don't drive the car much, so I've related this issue to just a bug that would go away with some extended driving or perhaps something that would be fixed by general maintenance. At the end of September I took a long trip, and the entire time the car had this issue. Anything below 2 grand, any gear, light or heavy load, and the car hesitates. It's almost like a stutter the way it acts, but it's not throwing any codes. Above 2 grand, no problem, runs like a scalded dog.

Before I went on the trip, I changed the plugs and ran a can of BG 44K engine treatment in the car (awesome stuff, cleans valves, fuel injectors, all kinds of stuff.) The wires are in good shape, the fuel pressure is strong, no fuel leaks or anything like that. Keep in mind though that the car did this hesitation before any of that work was done, so it's not related to the engine treatment or the plugs, and those two things didn't fix the problem.

The car has 89k on it and it has never had an optispark or a water pump. I'm thinking that the opti is the problem, but the things I've read in the past (if I remember correctly) is the opti just goes out, not causes a problem like this, and certainly not for an extended period of time. The only thing the car has on the engine that is not factory is a set of Ford SVO 24# fuel injectors and an MSD 6AL (both items for the 5176 NOS dry kit that is on the car but currently not used). Other than that, it's an SLP loudmouth modified with two bullets instead of the SLP resonator, the cats are gone, a Global West Traclink torque arm, and Spohn poly ended adjustable LCA's and PHR.

Is there any way that the LT1 knock sensor could be picking up a false knock from all of the rearend modifications or perhaps the exhaust? The reason I ask that question is that all of that suspension stuff makes the rearend more solidly connected to the car, and I know the LT1 knock sensor is super sensitive anyway, so I don't know if it could be seeing all that chassis noise as a knock situation. If it is a knock sensor issue, do you replace the actual sensor or the chip in the ECM? If I do decide to use the nitrous again, is it safe to spray on a stock engine with the LT4 knock sensor with no tune?

[Z Ya Later]

I don't think it would be the suspension parts, as I'm running the non-adj. Spohn parts you are along with the SLP SFCs with no problem of false-knock. Have you scanned the car to see what the PCM is reading? Right off hand, from what you're saying, it almost sounds similar to the EGR problem I had. While this may not be it, just try to see if you can push the diaphragm on the EGR valve and make it move. It should move freely. If it doesn't, this might be a good place to start. Hope this helps.

[96FirebirdFormula]

how does it hesitate when you get on it or lean into it? or just cruising at steddy rpm

[MN6WS6]

I previously had an EGR code, replaced a hose, cleared the code, has not come back.

Hesitation is regardless of throttle position or load. Roll in, steady speed, hammer on it, same hesitation. 2g's, disappears. It almost acts like a stutter or a misfire, but it is not throwing a code for that, and it happens all the time. If you are below 2 g's for 2 seconds or 10 minutes, it stutters. It's also irregular, not like a dead cylinder or anything like that.

[96FirebirdFormula]

does it hesitate at lower speeds? 10-20 mph or 40mph-50mph+++?

Because mine feels like it's Hesitating when im going 40 to 50 mph cruising at steddy 1.5k rpm i found out other day one of my rims are bent when it was on balancer it was shaking.

when last time got wheels balanced?

[BrdWAtti2d]

May not be the opti itself but the cap and rotor. Yes when the opti goes bad the car just stops running. But if it's related to the cap and rotor the symptoms can be as you described. When mine went out the symptoms started as you described but gradually got worse to the point the car was un-drivable and was missing under all ranges. I think my opti was fine, but I replaced it anyway as a pound of prevention while everything was apart.

[Injuneer]

There is no single failure mechanism for the Opti. Just think about the things that can go wrong, and you realize its very possible for the Opti to cause minor problems for an extended period of time. Based on working with the Opti for 13 years, I've seen them fail in multiple ways. My own suffered from the off idle to 2000rpm stumble, then held steady up to 4500rpm, and SOMETIMES would cause the stumble again between 4500-5500RPM. They do not all fail totally.

Things that can cause a long-term failure:
-gradual degradation of the bearing
-partial blinding of the optical sensor
-gradual breakdown of the dielectric properties of the epoxy in the cap that separates the embedded flat wires that route the cap buttons to the correct plug wire tower
-changes in the amount of moisture trapped in the case
-gradual increase in carbon tracking in the cap
-slow erosion of the metal on the rotor tip and the cap buttons
-corrosion builup and damage to the harness connector pins
-gradual loosening of the rotor screws

The Opti can even fail to produce the high resolution pulse pattern from the optical sensor, and it will still continue to operate, and allow the PCM to operate the engine, often with no sensible change in engine performance. You would see a code for this, but not necessarily an SES light. The only things that would be likely to provide a sudden, instantaneous failure would be the loss of the low resolution pulse pattern (PCM shuts down the fuel system) or a shattered or loose rotor.

I would also look at the SVO injectors. I never liked how they fit into the intake manifold. They are shorter than the stockers, and I also felt like there might be a bit of air getting by the o-rings on the bottom. That would show up as elevated long term fuel corrections, and possibly a stumble. Try coating the holes liberally with grease to limit the possibility of a vacuum leak.

You also need to scan it for knock retard. That is the only way you will know if you are getting any. I doubt that suspension components (other than something like tubular front a-arms, with rod ends that attach to the K-member and could transmit vibrations to the right side motor mount, which is right next to the knock sensor) are causing false knock. I'm not convinced that exhuast noise, other than the pipe rattling against the frame, will cause false knock retard. But you will never know until you scan it.

What year is your car?

[MN6WS6]

Fred, it's a 1996.

I have a Carchip EX which allows me to do datalogging, but it only takes a snapshot every 5 seconds, not real-time. I can look at timing, short and long term fuel trims, and quite a few other things. If there's anything specifically I need to look at, let me know, and I'll set it up to datalog those things.

On the o-ring thing, I had considered replacing the o-rings due to the amount of time they've been in the car (since about 2001 or 2002 and since it sits so much). I don't know exactly what to look for though, as I highly doubt the stock GM o-rings would fit it. Are they the same over a specific year range or particular models? I work with a couple of parts stores pretty regularly through the dealership I'm at, so if I have some kind of general range to work with, we could narrow it down.