OK, car is a 1989 Firebird 2.8L. Mostly stock, 238000 well cared for miles.

Car runs strong and everything works for the most part. EXCEPT:

Problem exists in both open and closed loop. At slight throttle (3%-25%) while accelerating the engine has a tendency to "hold back", almost like its rich or lean, like there isn't quite enough power there. If I keep the throttle held at the same point RPMS either increase or decrease at random. It never gets bad enough to stall, but its noticeable. Adding throttle almost always resolves it quickly. Sometimes it gets into a phase where the RPMS oscillate up and down every 2-3 seconds. This is usually when accelerating in a parking lot or a small local road where I don't really want to gun it.

I would describe idle as "fair" and it seems like it tries too hard to idle. Its always around 600-750, but it moves around. When I do below its a solid 800. Idle dips way low while braking, down to 400-600, but usually recovers at a stop.

I can fix it.... I unplug the MAF and the car uses predetermined values for the fuel mixture. However I know that this is hurting economy, its just that the car runs SO much better with the MAF unplugged. The hesitation is gone, the oscillation never happens and the car idles just about perfectly.

I have a modified air intake its basically deleted the air box and silencer, its PVC pipe from the rubber flex part of the original intake at a 90 degree bend, then a rubber spacer, the MAF, another spacer (with the air intake charge temp sensor placed in the airflow), then a cone type K&N filter.

I've also bypassed the throttle body heater.

This is a new ACdelco MAF, car has a new TPS (set correctly with a voltmeter), clean IAC and EGR passages, no vacuum leaks, in fact all new hard lines and convered with small washer fluid lines at all the joints, functioning EGR (in that I apply vacuum to the EGR valve and the engine stumbles), at least one of the CTS is working (my coolant gauge works), VSS is working too as the cruise control works.

Clean intake, no idea of the condition of the fuel injectors or the fuel pressure at the rail, I've replaced the MAF relay/burn-off relay and fuel pump relay and changed the brittle broken connectors for those. They are now making good contact.


While in closed loop and idling at a light the car also has this really odd habit of stumbling and occasionally stalling every 25 seconds. (to the microsecond. Its like clockwork). Its random too, I've gone weeks without it happening, but this past week it seems every time the car is warm and I come to a light. It happens in Park as well in the parking lot, but I can't MAKE it happen on command. When I shor the diagnostic terminal the SES light indicates open loop for the half second that it stumbles, then back to closed loop for another 24.5 seconds.

(maybe I have closed/open mixed up, but I'm referring to the mode that the ECM IS listening to the sensors. I've verified the mode by shorting the diagnostic terminal and watching the SES light. (LONG pauses between flashes, the occassional shorter blinks and longer blinks indicating the ECM is metering fuel. Immediately after startup the SES light blinks much more quickly indicating "open loop".)

Any ideas here? I'm willing to bet the fuel injectors are fine, since when the MAF is unplugged it performs perfectly. My thoughts lead to the other sensors, like the CTS and know sensor and whatnot. I bet I'd be able to find the problem right away if I were datalogging.

 

try swapping out the ECM, but my money is on a ground somewhere. make sure all your grounds and ground straps are intact/secure.

 

Gm states that on a EFI motor the tolerances of resistance on ground wires is very low... Your efi system needs between 13.8 and 14.1 volts to run properly. The biggest thorn in the side is the ground wires. Constant heat and weather adds a considerable amount of resistance. I hope this helps

 

Hey if thats the case than I'm screwed to begin with. Currently have a Walmart special battery that has 3 years on it now. 2 yr old alternator too, but its an advance auto parts special and I had to have it exchanged 3x before I got one that actually worked LOL.

When running the A/C volts dip under 11.5V sometimes. I've had to wait a few minutes for my battery to gather enough charge to turn the starter. And what you say about constant heat and weather adding considerable resistance is absolutely true.

Important things to bond to the chassis: ECM, Engine block, Battery neg terminal, Accessories (esp the alternator).

I think once I get a new (good) battery and a larger alternator I'm going to add a second setof grounds and not worry about what is currently there, after all another ground wire isn't going to hurt anything at all.

Sound like a plan?

 

as long as you get a alt that is an OEM replacement you should be set. I think the average for EFI is 130 amps... I use a V6 alt on my 4bbl 350. also it doesnt hurt to replace your grounding straps or even add new ones. check resistance on the wires going to your alt. Its an easy and cheap fix. you can have more than one grounding strap grounding your engine to the chasis.

 

Just a question, when was the last time you oiled your K&N?

I see it quite a bit where people over oil and then the oil gets sucked into the MAF and ruins the Hot Film/Wire.

Also, THe Temp sensor that runs your guage is not the one the computer uses. The one for that is the 2-wire sensor under the TB, the wires should be Yellow and black. You can check this one by using paperclips to backprobe the wires at the sensor and check the voltage between the 2 wires. Cold, you should be somwhere around 2v and hot you should be <1v.
----------
Something else I just thought of, where exactly is your IAT? Did you actually mount it IN the MAF? Like, drill a hole in the side and insert it? or is it infront of the MAF? The MAF uses the IAT information for heating the Hot Film and if it's reading air that is behind it, it'll be incorrectly calibrated, you'd be suprised at the difference 3 inches makes...man, if I had a dollar for everytime I heard that...lol

 

Problem existed as long as I've had the car, and I put the CAI in the day I got the car. Last time I oiled it was 8 months ago, and I replaced the MAF 2 months ago. I honestly thought the combination of fixing the connectors on the relays on the firewalls and the relays themselves, along with a new MAF would finally solve the problem.....

Yup, thats why I said at least one of them is working LOL

I never checked the CTS voltage, but its certainly something I will do now. I'm aware that CTS has a LOT to do with the cars operation, never knew where it was LOL. I'm almost positive I've seen those wires when I replaced the thermostat. Am I right that its not under the plenum, but sort of in that cavity that the water neck is also near under the TB on the front side of the plenum?

IAT is in front of the MAF. Its directly behind the K&N inserted in a hole I made in the rubber spacer. It extends into the edge of the airstream.

Hey thats a thought, the rubber itself might hold a ton of heat, wonder if its getting an odd reading since its in contact with that. Originally it was in the airbox which was plastic, and I know the camaros with the dual snorkel have it in the plastic snorkel... Hmmmm time to go check the volts on that thing....