Camaro99SS

iTrader: (4)

I've been having this problem for awhile now but am running out of ideas. It is most noticeable when I'm going around a turn in 2nd and give it about 1/3 throttle or more to accelerate. The car hesitates for about a second then takes off accordingly. It is also noticeable when taking off moderately and when I shift to 2nd, it will do this a little as well. It's as if the car has slow reflexes. I've replaced the TPS twice now, with the latest unit being a Delco brand. I did check the throttle cable, and it actually feels about right. There may be a small amount of slack, but if I tried to adjust it, it may make the car idle high.

Everything else is fine. Fuel pressure is around 60psi, the o2 sensors (Bosch 13111's) are about 5k miles old, the fuel filter is new, the MAF hasn't been messed with, and there are no codes being thrown. I also put new TR-55's in 2k miles ago with a .055" gap. Anyone else have this problem?


Jason

JoeyGC5

iTrader: (24)

What rpm range?

muncie21

iTrader: (14)

What do your O2s look like when this happens? A lean condition will cause the car to hesitate.

Camaro99SS

iTrader: (4)

It happens just about anywhere in the rpm range and at least in the first 4 gears. Sometimes it's bad enough to where it makes it tricky to drive the car smoothly when shifting gears. Like I mentioned above, the best way to describe it is the car has slow reflexes getting on and off the gas pedal.

As for the o2's, I can't answer that question. I haven't used a scanner to see what they're reading. They are Bosch 13111's. However, I have noticed one thing about the Bosch's. If the car is in closed loop and I turn it off for ~15minutes and start it back up again, the car will hesitate badly and sputter like it's not getting enough fuel the first time I accelerate. If the car is doing this, I can let off the gas for a second then get back in it and it will be fine. Again, it has only done this since I put the Bosch o2's in about 6k miles ago. Maybe that problem is tied to the bad throttle response all the time?

Jason

muncie21

iTrader: (14)

Just for grins, unplug your MAF and see what happens. This will put your car into SD mode and eliminate your MAF as a potential source of problem.

Camaro99SS

iTrader: (4)

Doh!! Never thought of doing that. I'll give it a shot. It will throw a code because it always did it when doing a Seafoam treatment. That won't make it run weird initially will it?

Jason

ringram

When did it come on? Any changes just before then?
If you are just getting into your throttle more these days it could be Burst Knock Retard?

muncie21

iTrader: (14)

It'll take a bit for the Long Term Fuel Trims to adjust to the running on just the VE table. But unless you are way out of whack, it shouldn't be that noticable.

Camaro99SS

iTrader: (4)

Well I've driven it around in Speed Density mode now, and that pretty well nailed the problem. Everything is fine now except it did throw a code after about 4 miles, but it's not causing any mishaps. I guess the MAF is just dirty or maybe needs to be replaced?

Once the PCM transitions over to the VE tables, will it be ok to do any WOT runs? I'm getting 60psi of fuel pressure and the air/fuel with the current tune is 12.8/1 on a wideband dynojet. I'm not familiar with how SD works. I know with the MAF hooked in, the fuel injector spray patterns are adjusted to spray the appropiate amount of fuel, making a fuel pressure regulator useless for adjusting the air/fuel mixture. I know my friend's 90 454SS truck with the stock TBI with no MAF needed fuel pressure adjustments via a regulator to keep the air/fuel in check. Is this how it would be with my car running in SD mode? Sorry for all the questions, but I'm a noob with EFI tuning.

Jason

muncie21

iTrader: (14)

Could be either. If this just started out of the blue, I'd say dirty or defective. If it happened after a mod, you may just need to calibrate it.

You'll need to get a WB (or dyno it) on it to be sure. In SD, the VE table tells the PCM how much air is coming into the engine and adjusts the injector pulse width accordingly. This table may be OK for part throttle driving, but could be off in the upper RPM/MAP ranges. When I say off, this means you could either run rich, lean, or OK...it's all a guess until you get it checked out.

I'd spend the $80 on dyno time instead of 800 on new pistons...if it were me

Camaro99SS

iTrader: (4)

Yeah that's what I was thinking. A local shop has a dynojet with a wideband, so I'll get on there soon.

The problem has been around for awhile but has become progressively worse the last couple months, although none of my mods had any adverse effect. That leads to my next question. Should I decide to ditch the MAF and convert over to SD via a tune, are there any drawbacks to running SD compared to a MAF, especially with a big cam and future engine work (not going FI though)?

Jason