phrogs

The car is stock
I changed the plug wires with a newer set of used 95 lt5 wires, spark plugs ac delco rapid fire #5's gaped to .035 and the stumble seamed fixed a little bit but it is still there...

Is there any way I can check the coil packs without pulling the plenum? like pull the wire and put a extra spark plug I have in the boot and start it up and see what the spark looks like?

I just dont want any down time with the car if they are bad and I need to replace them and then my car will be all apart its my daily driver :thumbs:

How can I test my plug wires to see if they are working properly?
I have a brand new set of taylors I bought they are sitting in my garage
anyway any ideas are welcome

johnny





[Modified by phrogs, 11:03 PM 8/29/2003]

Aaron71771

I don't think you can get a "solid" test of the coil packs until you pull the plenum. Its about a 2 hour job to pull it, test the coil packs and reinstall it.

Phil DeJohn

John, By any chance have you changed the fuel filter ? I had the same thing on my 92, Injectors, coils, wires all that stuff checked out good.I had the original filter, I changed it and that was it. :seeya . Phil.

phrogs

I havent changed the fuel filter
I didnt even think about it..I think ill do that this weekend thats a easy one!

the plenum pull is no problem I just dont want to pull them check them and find them bad then put it back together and wait for the new ones to show up then pull it all again that would be double the work..

of course I could have just changed them when I had it all apart the first time but that would have been the smart thing to do :crazy:


[Modified by phrogs, 10:25 AM 8/29/2003]

VetBoyZR1

Phil - Thanks for reminding me - I had the same problem and Frank Urbinati said the same thing. Just ordered my filter part #25121792 $9.27. If I see you at the track tomorrow I want to talk about that Vette Drs deal again!!

Aaron71771

When I had a bad coil, I could feel the stumble in the low RPM quite frequently. I also have the occassional well pronounced miss.

The effects of the bad coil could be felt thru the entire RPM band, not just the low side (tho under 3500 RPM is where they effects were most pronounced).

Rkreigh

if you end up pulling the plenum after all else fails, have the injectors flow tested, and you can pull them out, leave them attached to the fuel rail, turn the pumps on (test wire) and see if they are leaking. the stumble could also be caused by a rich (leaky injectors) or lean stumble (clogged, or not working at all!!) condition.

do the simple things first, and hope for the best!!!

phrogs

The injectors have already been flow tested and they are all good to go sgc did them.

FastZR1

Drive the car a bit harder. Run the crap out of it for a day or so. Or come with me when we hit a track. There is an autocross up in VA Beach on the 20th. 14 runs of 7000+ RPMS. That will clear up the stumble, right RPM?

VetBoyZR1

No - I've been trying that. There is more to it! That used to work back in the gummy carburetor days, but not with todays engines.

phrogs

Ed I think I drive it pretty hard at least as hard as I can and still hide from the state troopers :D

I cant go racing with you because Im leaving for yuma AZ on the 19th good old WTI

Patand

Take a look at the MAP sensor line, mine was a bit clogged with oil. I cleaned it with wd 40 and toppsy. Also do a leak test as well. I had a stumble and the car would miss at low rpm high load...seems the above helped me..
:steering:

PA

VetBoyZR1

All of the fixes you guys are describing sound like things that would cause this problem all of the time (clogged vacuum hoses, filters, injectors, coils, etc). Why would my car run good all the way to the track, run good for the first 3 or 4 runs, and then all of a sudden stall or stumble on the fifith start, then run OK after reving it up? Seems like a software gremlin to me. Any thoughts?

Chris@VetteFinders

This is my story as they say. I hope something here may help you:

purchased my 1990 ZR1 back in January and have been battling a hesitation problem since then. I was finally able to solve the problem. (Note: I had the plenum and injector housings ported in the interim, but the hesitation did not change at all). I spent hours and hours going through ZR1 net archives, forums, the service manual diagnostic charts and even replacing a few "may as well try it" parts. In my search for the cure, I found that this is a very common problem, especially among 90-92 ZR-1s. I hope my information may help someone else out. If you are pulling the plenum, I suggest you go ahead and try this fix.
The symptoms were as follows.

When I accelerated mildly between 2,000 and 3,500 rpm, after about 1-2 seconds with my foot on the pedal, the car hesitates badly. Just for a split second, like it is either momentarily rich or lean. It only does it with the power key on (secondary system activated). With an aftermarket chip in, the problem is MUCH more pronounced. So much so, that you can feel the backlash in the drivetrain. With the stock chip, it was fairly mild. If I accelerated very slowly, it would not do it. If I punch it, pressing the throttle immediately to the floor it would not do it (at least I could not notice it). When revving the engine in neutral, no evidence of a problem. Power through the rpm range was as it should be.

The first thing I tried was removing the plenum and seeing if the actuators were installed "backwards". As you all know, the jury is still out on which is the "right way". Mine were "backwards". I switched them around and found no change. Next I grabbed the service manual and went through the charts.

Tested the injector resistance with them in place. All good.
replaced fuel filter
Injectors sent to cruzin performance. They tested within parameters.
replaced my secondary vacuum system. It was holding vacuum.
replaced vacuum solenoid
checked fuel pressure. The pumps were putting out on the low end. Replaced both pumps, same problem
checked TPS
checked fuel press. regulator
replaced spark plugs
drove without intake on
checked coil packs
checked plug wires
checked MAP sensor.
checked IAC
checked vacuum leaks.

At this point I had tested everything in the service manual for hesitation and surges that exhibit the problem with the power key on.

I then borrowed a diacom and hooked up a laptop. Well, the results finally showed me something I could use. I found that the exhaust on the left and right sides were going lean during the moment of the hesitation. Normal O2 sensors do cycle back and forth between 300-800 or so mv at idle. But during acceleration, you should see somewhat of a symmetric exhaust O2 profile. Mine did not show that. Here is a screenshot of the Diacom output. http://vettefinders.com/zr1/diacom2.jpg Notice where the vertical line is. At that moment I had the hesitation. The corresponding values are on the left side. Throttle position was 82% and accelerating. The O2 sensor was getting richer, then dropped to 82 mv during the hesitation. Very lean.

Now I suspected that something was causing the cylinders (both sides) to get a burst of air at about the time the secondary system was activating. I thought it had to be either the butterflies opening or something in the throttle body.

I removed the plenum (for about the 10th time) and took a closer look at the actuator operation. If I hooked up a vacuum pump to the actuators, they appear to operate very smoothly. I put my finger on them to see if I could feel any indication of binding while they were opening the butterflies. I could not feel anything. Next I pushed on the actuator arm and on both of them, I felt them bind while they were opening. I would think that applying vacuum to the actuators and NOT pushing them with my finger (not applying vacuum) would be perfectly representative of their actual operation. I flipped the actuators again and swapped them out with new ones. The same thing occurs on all four. I then decided to put the pin from the butterfly arm into the hole of the actuator arm closest to the body of the actuator. This does two things. It makes the normal deactivated (power key off) position of the butterflies open about 15 degrees and also reduces the distance the arm has to travel. In this position, the actuator arms operated smoothly when vacuum was applied as well as when pushing the arm with my finger. The amount of vacuum required to open the secondaries fully before and after I made this change was 7-8 in. Hg.

I reassembled and went for a test drive. What do you know, no stumble or hesitation! All I feel is a little acceleration when the secondaries kick in now. So, I am left with drawing the conclusion that either the actuators were sticking (which it did not appear to be the case under vacuum operation) or that when the butterflies opened they were popping open and causing the lean condition.

I took a look at the diacom results after my change and I do not see a lean condition (as I expected from drivability improvement). My next step is to get a new chip calibration. The computer appears to be able to handle the calibration for leaving the butterflies in this position, according to the diacom results (02 sensor and block learn) .

A26B

Johnny,

If the coils and wires are in good condition, it is not necessary and is, in my opinion, detrimental to gap the plugs @ .035. Spark is hotter @ .050. Admittedly, a closer gap will fire easier with a weak ignition component, but the objective is to get it all working properly.

You can check coils through the wires (paired cylinders) for proper resistance, but I can tell you from experience that you can have a coil that will put out very weak spark on just 1 cylinder only. A coil does not necessarily fail completely or act the same for both of the cylinders on a coil. A coil problem of this type can also be intermittent.

A weak spark on 1 cylinder will result in a low rpm miss that is most noticable under load, such as gradual acceleration in 6th gear from 60mph. Check the spark by putting a spare, good plug in each plug wire and hold it about an inch away from the cam cover at an idle. Good spark will look blue, weak spark will be orange-yellow. If you are certain the plug wires are good, then the culprit is the coil.

If your oxygen sensors have never been replaced, I highly recommend new ones. Autozone sells Bosch (identical to OEM) for $48 each. Leads are a little longer than OEM which makes it easier for cars with headers. In closed loop after engine warms up and under 3200 rpm if memory serves me correctly, the A/F is managed by O2 sensor input. This is the range where you appear to be experiencing the stumble/miss. You have checked your injectors but they may be getting an improper signal.

Semper Fi,

Jerry




[Modified by A26B, 7:45 PM 9/3/2003]

phrogs

Chris the last time I pulled the plenum 2 weeks ago, I replaced the entire vacum system except the vacum pump that I still havent changed, everything else has been replaced, I did do that little trick with the actuators I drilled the second hole out just a little bit and I repositioned the arm on to the closer hole so the plates are now open just slightly now.


I now have my new napa silver fuel filter
Im going to change the filter tonight along with my oil pressure switch its leaking!

Jerry, I had heard about going to .050 but I went with the manual I guess It wouldnt hurt to gap them to .050 and see what happens, Ill do the spark check tonight and see what the look like.
Also I have no idea if the OXY sensors have ever been replaced if I was a betting man Id say now but I went to auto zone and they dont have any oxygen sensors in stock right now so ill have to check on that later on.


johnny :thumbs:


[Modified by phrogs, 6:07 PM 9/3/2003]

Chris@VetteFinders

Can you post some more details on the symptoms? Power key on only? only at certain rpm? only at certain throttle position?

Have you already tested the injector resistance and plug wires?

You want to borrow my diacom?


[Modified by Chris@VetteFinders, 7:12 PM 9/3/2003]

VetBoyZR1

To those of you who swear by replacing the fuel filter, I was thinking of you because my new part came today and it says this right on the box:

"Regular replacement with an ACDelco fuel filter helps avoid hard starting, rough idling, and stalling due to fuel contaminents."

Man, this is the biggest fuel filter I ever SAW!!! :lol:

95ZR1#418

Jonny...

Let us know what you think your fix was when you find it. :chevy :chevy

phrogs

Chris My power key is always on and I havent tried it with it off I guess I should do that and see if it is still there, I had my injectors checked at SGC and I had 5 of them that were bad, replaced the bad ones with 5 that a guy sold on the zr1.net that had also been tested, But I have not checked my spark plug wires I have a brand new set of Taylors siting on my shelf if this stuff dosent fix it Ill move on to more drastic measures.
Im going to try to tear into it tonight after work I had to go to the grocery last night so I couldnt get to the car fuel filter is first then im going to check the spark at each cylinder.

About the diacom is that hooked up through a laptop? or does it just plug into the under dash conector?
and I will update on my sucess or failure :thumbs:


[Modified by phrogs, 7:41 AM 9/4/2003]