bad stutter at 2000 rpm, only power key on

[Spanky1965]

:dontknow:hello all, newbe here, I am sure this has been covered but this car is driving me crazy.

Runs perfect under normal power, but turn on the power key and idle goes up, and at exactly 2000 rpm's at part or full throtle it sounds like it stutters, bad, clears up at 5500 and pulls hard to 7500.

:dontknow:I have changed the fuel filter and pumps. new plugs. From what I have read in other posts it sounds like bad injectors, but it realy feels like a carb with the choke sticking. I was wondering if the 2nd ports are not opening. I am in Springfield Missouri and need help

[Spanky1965]

just wanted to add the car sat for 3 years before i got it, it has 30,000 miles, and had the vacum line that runs to the comp on the drivers side disconected.

[tccrab]

Try this test:
Turn the Engine Power Key to off.
Run it hard, all the way up to 7000.
Turn the Engine Power Key to On.
Do the same thing.
You should notice a significant difference in power between the settings.
If there's only a slight difference or no difference at all, your secondaries system is suspect. Injectors, secondary actuators, secondary vacuum components will all cause symptoms like these. The fun part is figuring out which.

Here's everything you should ever need to know about your secondary port vacuum system:
http://www.corvetteactioncenter.com/forums/zr-1-general-technical-discussion/14272-1990-1995-zr-1-secondary-port-vacuum-diagnosis.html

Good luck!

TomC
"Crabs"

[Spanky1965]

thanks Tom, the funny thing is there IS a big diff, not even close. also the vac pump runs for 3 sec and does not cycle on and off so i am assuming no vac leaks

[tccrab]

thanks Tom, the funny thing is there IS a big diff, not even close. also the vac pump runs for 3 sec and does not cycle on and off so i am assuming no vac leaks

Ok, so no huge vacuum leaks. That's good.
Narrows it down a tick.
Your secondary actuators could be sticking and then opening later than they should. That would cause a mid rpm bog as the fuel mixture goes rich and then once they're open, the power comes back as the mixture leans out.
It could be fuel injectors, if you've got secondary fuel injectors that aren't flowing like they should, it could cause a bog.
Coils can cause stumbling problems, check out this 3 year long horror story:
http://www.zr1netregistry.com/forum/showthread.php?t=1777
Doesn't sound like your problem though, my first guess would be fuel injectors, secondary actuators next.

TomC
"Crabs"

[Spanky1965]

I agree, but can you test the secondary actuators without removing the plenum?

[LGAFF]

Is the 2ndry MAP sensor under the ECM connected?

[Spanky1965]

yes it was diconected when I got it, but my friend did not drive it like you or I would, so not sure he even noticed. I have conected it

[tomtom72]

I agree, but can you test the secondary actuators without removing the plenum?

I'm not sure that this would work. The secondary actuators are twined and the line off of the left bank actuator is T-'ed to go to the 2* MAP sensor(under the ECM bracket). Maybe if you pull the line at the MAP and apply vacuum to it you might be able to watch the actuators' arms thru the gaps in the plenum runners.:dontknow:

I'm not sure if that will work.:o

[Kb7tif]

I bet your secondary throttle plates are not opening up or vacuum/injector problem. This stuff is generally easy to fix if you understand the system.
Here is a start.http://www.corvetteactioncenter.com/forums/zr-1-general-technical-discussion/14272-1990-1995-zr-1-secondary-port-vacuum-diagnosis.html

[LGAFF]

I agree, but can you test the secondary actuators without removing the plenum?



You can peer in between the runners and watch them open.

[Spanky1965]

how without starting engine?

[Jeffvette]

how without starting engine?


Jumper pin C17 on the ecm harness to a ground with the key on.

[Kb7tif]

Jumper pin C17 on the ecm harness to a ground with the key on.

Pink wire. Have you ever had the plenum off? On mine that was all working and the Secondary throttle linkages were broken off even though the actuators were working, the plates stayed closed-:wave:

[Dynomite]

:thumbsup:Pink wire. Have you ever had the plenum off? On mine that was all working and the Secondary throttle linkages were broken off even though the actuators were working, the plates stayed closed-:wave:

OK...I corrected my previously assumed location of pin C17. My screw driver (I am assuming) is now pointing at the corrected location (that wire is pink with black strip on my 1991) :D

Thank you tccrab for the information :thumbsup:

What are the pinouts for the ECM (which ECM is it?) for a 1991 ZR1 :???:

http://i287.photobucket.com/albums/ll142/dynomite007/1991ZR1ECM-1.jpg

[tccrab]

How to check the secondary actuators without removing the plenum:

http://www.zr1specialist.com/HAT%20Web/articles/Verifying%20Secondary%20Actuator%20Operation.pdf

Lots more interesting and useful reading at Marc Haibeck's website:
http://www.zr1specialist.com

TomC
"Crabs"

[Spanky1965]

thanks guys, lots to check out

[Spanky1965]

OK, by grounding the pink wire with the black stripe I popped the ecm fuse and car does not start...LOL

[Jeffvette]

OK, by grounding the pink wire with the black stripe I popped the ecm fuse and car does not start...LOL


You hit the wrong wire then.

[Spanky1965]

duh...LOL (just kidding)

[Spanky1965]

Picture 1 shows how the pin tool can be slipped through the weather pack seal and into contact with the connector pin. Note that the seal is not pierced by the pin. The pin is slipped between the soft sealing ring and the wire. Pin C17 is on the green ECM connector. The wire is pink on ’90 LT5’s and pink with a black stripe on ’91 – ‘95’

should have read this first...LOL

[Spanky1965]

OK, so I did that and everything works, even did it when running and checked the vacumm to the comp, all good. so one question, instead of removing the system why couldent you just run with it grounded"?

[Dynomite]

OK, so I did that and everything works, even did it when running and checked the vacumm to the comp, all good. so one question, instead of removing the system why couldent you just run with it grounded"?

Ha....I am trying to learn..being a good student and you are saying you blew the ECM fuse...I did not know there was such a thing :D

Why would you now try to "jerry rig" your 2000 rpm stutter when you have Jeffvette, tccrab, and Kb7tif working on it for you.....believe me...they will get it running like it is suppose to ;)

[Spanky1965]

sorry not trying to do that, I was only asking since I have seen other posts about removing it...anyway so does it sound like injectors to you guys? and if so, what do I buy?

[Jeffvette]

You have a fuel pressure gauge? Voltmeter?

[Spanky1965]

yes and yes

[Jeffvette]

What is your fuel pressure drop when you cycle the key off? Pressure bleed off rapidly?

And start ohming your injectors.

[Spanky1965]

ohming the inj??

[Jeffvette]

Yes, taking a ohm reading of the injectors. The coil on the inside.

[Spanky1965]

I need to rmove plenum for that? if so why not just change them?

[Jeffvette]

No, you can do it with out removing the plenum. Have a service manual?

[Spanky1965]

no, amazon?

[Jeffvette]

www.helmsinc.com or www.ebay.com & I would imagine yes at Amazon as well.

[billybaloneey]

no, amazon?

eBay has them from time to time also...most of them offered there will be used and much cheaper than new.

[tccrab]

How to check injector coils without removing the plenum:
http://www.zr1specialist.com/HAT%20Web/articles/FI%20Resistance.htm

TomC
"Crabs"

[Spanky1965]

thanks guys, but since I have an all original 90, that sat, does not stutter on normal power, and the 2nd port system is working, then am I wrong to assume it must need new injectors?

[Dynomite]

Seems to me that if the The ECM drives the secondary vacuum solenoid valve by grounding the electrical circuit that maybe the ECM is not grounding like it should if the issue is with the ECM or input to the ECM regarding secondary opening ;) Except here the issue apparently happens exactly when the secondary kicks in (indicating the secondary is in fact operational).

I guess you can open secondary at 40% throttle around 2000 rpm from the diagram below.

So....I guess you are assuming that since it stutters at 2000 rpm that it IS in fact caused by opening secondary (40% throttle) which the injectors/fuel supply are not functioning causing the stutter with the change in air flow? Just trying to learn the diagnostics.

Kb7tif provided this:
http://www.corvetteactioncenter.com/forums/zr-1-general-technical-discussion/14272-1990-1995-zr-1-secondary-port-vacuum-diagnosis.html

http://i287.photobucket.com/albums/ll142/dynomite007/lt5graph-1.jpg

[Spanky1965]

see thats what I was thinking, that is why i asked before could I just ground it and try driving her?

[Spanky1965]

oh and the ecm has a sticker on the back of it that says it has been replaced by GM

[Dynomite]

oh and the ecm has a sticker on the back of it that says it has been replaced by GM

Ha...ya...but after thinking...it would not stutter unless something happened at 2000 rpm....which as you can see...secondary can open at 40% throttle around 2000 rpm and since you do have a stutter then....secondary prolly DID open correctly leading to Jeffvette diagnostics of injectors and fuel starvation (too much air).

If the secondary did NOT open at 2000 rpm there would be no reason for a stutter right at that moment. Seems to me it would run poorly at rpms above 2000 also after the secondary is open.

Or.....the secondary did NOT open and the injectors gave it more fuel anyway at 2000 rpm (too much fuel).....LOL...........I am not sure how that works with the secondary and primary injectors. If I ever run into this problem, I am going to just disconnect the injectors and open up all primary and secondary, stick two Holly Three Barrel Carbs on it and GOOOOOOO!!!!!!

I am like you....just trying to figure out how the experts are diagnosing this so I know for future reference ;)

[tccrab]

Typically, bad fuel injectors cause symptoms such as a missfire at idle, rough running at a constant RPM and a loss of power when the secondaries are opened. It all depends on which injector(s) go bad.

When a fuel injector's internal coil goes shorted, it usually causes the injector to stick in one position. That position could be wide open fuel, partial fuel, or no fuel at all.

A bad primary injector will cause misfire at lower RPMS that goes away when the secondaries open.

Secondary fuel injector(s) failure will cause a loss of power which usually manifests itself when you can't feel much a difference between Full Power mode and Low Power mode on the Valet Switch.

Your situation as you describe it, sounds like the engine goes either rich or lean during the transition time between High Power mode and Low Power mode, which clears up as engine RPM increases.

TomC
"Crabs"

[Dynomite]

Typically, bad fuel injectors cause symptoms such as a missfire at idle, rough running at a constant RPM and a loss of power when the secondaries are opened. It all depends on which injector(s) go bad.

When a fuel injector's internal coil goes shorted, it usually causes the injector to stick in one position. That position could be wide open fuel, partial fuel, or no fuel at all.

A bad primary injector will cause misfire at lower RPMS that goes away when the secondaries open.

Secondary fuel injector(s) failure will cause a loss of power which usually manifests itself when you can't feel much a difference between Full Power mode and Low Power mode on the Valet Switch.

Your situation as you describe it, sounds like the engine goes either rich or lean during the transition time between High Power mode and Low Power mode, which clears up as engine RPM increases.

TomC
"Crabs"

Are the secondary injectors and primary injectors the same on a 1991 ZR1? What are the RC #s for primary and secondary injectors for a 1991 ZR1? Where is the best place to get RC injectors?

[Jeffvette]

RC 205 or RC 210. You can only get them from rc directly. www.rceng.com

They will cut the injector for the differences in the primary and secondary.

I personally recommend accel injectors.

[Dynomite]

RC 205 or RC 210. You can only get them from rc directly. www.rceng.com (http://www.rceng.com)

They will cut the injector for the differences in the primary and secondary.

I personally recommend accel injectors.

What are the Accel #s for secondary and primary (1991 ZR1) and where can I them?

[Spanky1965]

So Jeff can I ground the pink wire and drive the car to see if it helps?

[tccrab]

What are the Accel #s for secondary and primary (1991 ZR1) and where can I them?

Jeffie'll get 'em for you.
He's the MAN!!!

TomC
"Crabs"

[tccrab]

So Jeff can I ground the pink wire and drive the car to see if it helps?

Hmmm..... Good question.
Better wait for Jeff to answer this one.
I'm guessing it might cause a lean condition, but maybe not.

Tomc
"Crabs"

[Dynomite]

So Jeff can I ground the pink wire and drive the car to see if it helps?

I am assuming you are then confirming that the ECM is good or bad? If it helps you are assuming that the secondary is not opening when they are controlled through the ECM? I do not know how this is different than guys/gals tying their secondary open. I would assume it would start out lean but I guess they are also modifying the secondary injector control.

I am just trying to figure how this all works ;) It just takes time for some dunderheads like myself :D

[Dynomite]

Jeffie'll get 'em for you.
He's the MAN!!!

TomC
"Crabs"


:thumbsup::thumbsup:

[Spanky1965]

well my thought was if you can remove the system why cant you run with it grounded open???

[Jeffvette]

well my thought was if you can remove the system why cant you run with it grounded open???


It will signal a fault code. And what are you going to be doing for extra fuel, when the secondaries are not running?

[Dynomite]

It will signal a fault code. And what are you going to be doing for extra fuel, when the secondaries are not running?

You are talking secondary injectors? And they normally kick in when the secondary opens?

So if you grounded "open" the secondary butterfly, you will not get fuel untill the secondary would normally open at which time the secondary injectors would receive a signal to provide fuel also? I am assuming then grounding the secondary has nothing to do with the secondary injectors.

[Jeffvette]

You are talking secondary injectors? And they normally kick in when the secondary opens?

So if you grounded "open" the secondary butterfly, you will not get fuel untill the secondary would normally open at which time the secondary injectors would receive a signal to provide fuel also? I am assuming then grounding the secondary has nothing to do with the secondary injectors.

Or are you saying by grounding the secondary, you are not only opening the secondary but disconnecting the signal input to the secondary injectors?


When you are grounding C17, you are only taking care of the solenoid. The ECM controls the fuel pump and injectors as well.

So if you take one component and you are constantly grounding it, you will set a code.


Basically the ECM sees the demand for secondary operation. It then sends a signal to the solenoid, and the fuel pump and the injectors. For everything to function.

[Spanky1965]

ok so what should I do next? please

[Jeffvette]

ok so what should I do next? please


Refer to post 28.

[Dynomite]

I checked my primary injector resistances. I disconnected my battery and connected Inj-1 fuse side toward front of my 1991 ZR1 through digital ohm meter to ECM connector A (drivers side most left connector) A8, A3, A2, A7, A1, A12, A13, and A18. I got 14 ohms on 7 primary injectors and 6.5 ohms on one primary injector....OOPS. So....rather than check my seondary injectors without removing the plenum, I will remove the plenum and replace all injectors.

http://i287.photobucket.com/albums/ll142/dynomite007/Fuses1991.jpg

[Jeffvette]

:thumbsup:

[Spanky1965]

ok, 12.7-13.2 on all 16 injectors

[Spanky1965]

Fuel presure is 50lbs, turn off key goes to 48 and sits there for over 5 minutes. with running its at 42, when you stab the gas it jumps up to 50. drops back to 42 at idle, and if you turn off the car it creaps up to 50:dontknow:

[Jeffvette]

ok, 12.7-13.2 on all 16 injectors


Starting to get weak on the coil. Where they hot when tested?

[Spanky1965]

yes they were warm, not real hot, here is the strange part was running perfect on normal power before today, now after all this it runs like crap all the time, sounds like runing on 6 or 7 cyl???

[tomtom72]

While it's running badly did you notice if it got worse the longer it ran?

Also, while running badly, did you notice how the exhaust smelt? Rich, or lean?

I'm asking because if the injector coils are suspect their performance degrades with heat & time, and if they shut down you sometimes can smell the lean (or rich condition if they're stuck open) at the tail pipes.

:cheers:
Tom

[Ccmano]

This thread is becomming painful to read. Time to pull the plenum and replace the injectors. If it's a 90' with original injectors they will need to be replaced anyway, it's just a matter of time. I'd bet money it's the secondary injectors. I had exactly the same issue (bad studder key on only) Put in a set of Accels, problem fixed. You want 21lb injectors, AtlanticSpeed.com has them, about $550 a set of 16. I'll bet if you ohm the injectors directly you find at least one bad one.
H
:cheers:

[Spanky1965]

well thats what I was thinking, but I was thinking I should change coils also, but my concern is could it be a bad comp?

[Dynomite]

well thats what I was thinking, but I was thinking I should change coils also, but my concern is could it be a bad comp?

I have been following this thread and learning from Jeffvette and others ;)

I found one bad primary injector on my 1991 so pulled the plenum, injector housings, cam covers, starter, and about everything else on the top end :D

I will replace coils, injectors, plug wires, plugs, maybe some vacuum connectors. Jeffvette will powder coat and port injector housing and plenum :thumbsup: Everything under the cam covers looked in great shape with minor wear. Now to find all the parts and figure out where they go when I reassemble. Good ole Marie Calendars pie pans worked great for individual sets of bolts and parts :thumbsup:

Best of all was limey providing that ingenious suggestion of removing the drivers side engine mount nut and jacking the engine up on drivers side to remove drivers side cam cover torx bolts :thumbsup:

[Ccmano]

"well thats what I was thinking, but I was thinking I should change coils also, but my concern is could it be a bad comp? "

If your going to the trouble of pulling the plenum to change the injectors you might as well change out the coils and wires, replace the pvc valves and hoses, replace the breather box elbows and replace the breather box gasket (the bolts will be loose). It's a 20yr old car after all. Beats having to go back and do that again and again.
H
:cheers:

[Spanky1965]

Ok guys I am about to give up, anyone want to fly to Springfield Missouri and fix this thing? I will pay

[Jeffvette]

Ok guys I am about to give up, anyone want to fly to Springfield Missouri and fix this thing? I will pay


:hello:

[Spanky1965]

Jeff how do I contact you?

[Jeffvette]

Sending PM.