Idle Quality Issue
 

Hi gang. I'm having an idle quality issue in my 91 and would like to get some feedback from you all.

Idle quality was pretty bad a few months ago. Injectors were original and leaking down, the wires and plugs were all original as well, so I changed them all out. I threw in a new set of injectors, plugs and wires I got from Jerry, gapping the plugs to .035. I also found the pumps weren't holding pressure key on engine off and replaced those as well, and for good measure changed out the fuel filter. The pumps were only 6 years old.

After all that, the car has improved low end drivability, and pulls very hard. However, the idle is not very smooth and I can't seem to pinpoint what the issue is. Currently, with key on engine off, and FP2 fuse pulled, I get 50 psi of fuel pressure (I do have to cycle the ignition a couple times to get there.) With FP2 fuse back in, it shoots to 50 psi. When the pump stops, the pressure holds around 42. It then leaks down very slowly, perhaps 1/2 psi every 20-30 seconds. At idle, fuel pressure bounces back and forth between 41 and 42. There are no check engine lights and it is very smooth everywhere but idle. I put a new IAC in it just to check and that didn't make a difference. I also have a 1 Bar MAP on the shelf from another project, and swapped that out with the one in the car and that made no difference either. I checked the plugs and they are good, with no soot or oil on the strap or electrodes and color is pretty good.

I ohm checked the coils through the wires and got the following values:
5,8: 6.96K
1,6: 7.09K
7,4: 6.90K
3,2: 7.13K

According to the FSM, these ohm readings should be fine.

Any thoughts on why I can't seem to get a smooth idle? Many thanks in advance.

Re: Idle Quality Issue
 

Hello, and welcome to the ZR-1 madness!!

Sounds like your nibbling all around the idle issue. And, I could be wrong (and I apologize if I am, but I caution ya on (what appears from you post to be) throwing parts at a problem w/o really doing the "due diligence: "Never assume!"

Barring a bad check valve in one of the pumps, the very small leak down results you have - tho not ideal - wouldn't likely cause an idle issue or missing @ low rpm, IMO. Loosing several psi over a minute could cause problems, but usually is accommodated with other signs, e.g., fouled plug(s) and disparaging BLM results (balance).

You need to scan it and see what is the big picture. You'd be interested in what the O2 sensors are doing - are they working and are they balanced?
The BLMs will tell you if you have a fueling issue on one bank or the other (or BOTH). Are the cross counts reasonably the same or is one or the other sensor "lazy" (one O2 w/ 1/2 the cross counts of the other would indicate the sensor was well on its way to the trash barrel!)

Reading the plugs can give you a clue to whether a particular cylinder(s) are running rich or lean.

Is the injector dwell time on the two banks uniform?

Dominic (aka XfireZ51) recently wrote an interesting article describing the effect (or LACK thereof) injector flow abnormalities has on idle and low rpm response and over all drivability.

Food for thought!

Re: Idle Quality Issue
 

Thanks for the reply and the welcome to the ZR-1 madness...although I've had this madness since 93. I completely agree with you on throwing parts at it. The idle issue was just one issue, the other was hard starts which was being attributed to the system not holding pressure at key on engine off. It would crank and crank and crank. Upon checking the injectors and the pumps, I found them to be at fault and so I changed those out. This solved the hard start issue (I forgot to mention that in my original post). I was also experiencing a heavy hesitation right before the secondaries would open as I would ease into the throttle, but not to full throttle. I found the O2 sensors to be lazy. Changed those out and the hesitation is gone...in fact so is the feeling I used to get when the secondaries would open...it's much smoother now.

I'm happy with the results of changing out the parts I did, as the car responds a lot better at part throttle than it used to. I'm just at a loss for the idle deal. I'll look through your recommendations listed and go through the data to watch for those values closer.

Thanks again.

Re: Idle Quality Issue
 

Hey Cadillac.
Any updates? Any luck sorting it out?

Re: Idle Quality Issue
 

Hi Erik.

No, I haven't had much chance to look at it further. We had a car show last weekend that I took my Pro Street car to, so I was busy getting that car ready. I'll look at it, hopefully, this week.

Thanks for checking on me.

Re: Idle Quality Issue
 

have you check/replaced the TPS? the best closed throttle voltage is .53 volts and it should be near 5.0 volts at WOT.

Another point I would like to make and have made before, I always use GM parts, aftermarket parts are designed to "fit" multiple applications and can vary in what they read and how they deliver the information.

the LT5 runs on electronic input and is based off of the MAP sensor for all fueling, GM designed all of the programing off of very specific outputs and the small variations in aftermarket parts can collectively have an effect on the performance of the engine. IMO

John

Re: Idle Quality Issue
 

Hi John. Yes, my TPS shows .53 volts at idle.

Thanks.

Re: Idle Quality Issue
 

cadillac,

What injectors did u use as replacements? And when u say the motor doesn?t idle smoothly, can u describe more specifically? Has the motor ever idled smoothly? Is it stock?

Re: Idle Quality Issue
 

Spark plug gap was revised from 0.035 to 0.050, just to improve idle.

Re: Idle Quality Issue
 

Sorry, for the delayed reply here. I haven't been on for a while. The injectors I used are the FIC's from Jerry. A nickel won't balance on the plenum, and to be honest, I don't think it ever was able to do that. I might be able to get it, then it would shimmy and the nickel would fall over. The idle quality can be felt in the seat pretty predominately, and I tend to get some flywheel knock. Yes, the motor is stock.

It has been running terrifically in all areas but the idle. Today was a different story, though. Taking it out for a run today, it was fine going. The idle was the usual idle I've been experiencing and ran fine in all other areas. On the way back home, I started to have the "SES" light flicker on and off very quickly, the engine would miss and seemingly lose power all together and the "SYS" would flash in the cluster. This was happening intermittently all the way home. Power would restore when the SES light was out, but coming to a stop at a light the RPM would drop to about 350 and then it would want to die (did die once on the street).

Started it right up and was able to get it home where I could put a scanner on it. As I pulled it into my shop, the SES light came on and it died. I grabbed my scanner only to have it tell me no codes were found. It started right up and I scanned the data. My IAC steps were 144 steps at a coolant temp of 213.8. Desired idle was 662 and it was idling at 675, TPS was .54V and MAP was 1.27V. I was watching my O2 cross counts on both sides go to 255 then start over again at 1 and continue to count and block learn was 128 on both sides. To be honest...I have no idea what cross counts are and I don't know what's an acceptable value for the block learn. I do know the IAC steps are certainly an issue, as that seems like it's a very lean idle, I just don't know why they would be so high on a stock engine with stock tune.

Now, for the fun part. As I was waiting for the whole SES, SYS thing to happen (and it did), I was reminded about a post about 10 years ago or so, where someone in frustration, threw his hands up in disgust, and when his hands came down, they glanced across the computer and the engine died. While the engine was running, and before it died, I got out and tapped on the computer lightly. Sure enough, it sputtered and died. I duplicated the issue about 5 more times to be sure. So, besides the IAC issue, the computer seems to be on its way out giving me the loss of power, SES, and SYS. Not sure if that is the culprit for the idle, too, or not. I do have another computer and I will try to swap it out over the next couple of days.

Now for my questions: If an IAC is bad, would it sit so far open at 144 steps? I've read the threads about resetting the IAC, is that something that would just go out of tolerance over time? In regards to the ECU, my Z is a 91 and I think the computer I have is from a 90 Z. Will my prom still work in the computer from a 90 Z?

Sorry for the lengthy post.

Thanks.