Running rough/missing (94 ZR1)

[Rusty]

About 1.5 months ago I bought my first Corvette and really jumped in the deep end.. a '94 ZR1 with a good number of miles (~140,000 km) that, I suspect, hasn't been treated overly gently at its previous owners'. I found when driving it home after purchase (~200 km) that after running for a while and warming up, the car develops what feels like a miss. It kind of makes a stuttering/gurgling/popping noise, the car jerks around a bit, and it keeps going. No Service Engine Soon light comes on. It doesn't hesitate when accelerating and seems to idle well enough. We just put in new spark plugs, hoping for an cheap/easy fix, but that doesn't seem to have helped the problem much (if at all). I've probably gone through 2.5 tanks of premium fuel since purchase, so it doesn't seem likely to me that it's just some bad or old gas.

I've read a lot of posts on the forum about a similar sounding problem in 90-92's that are making me wonder if it could be the fuel injectors on my '94, but I'm not sure if there are other "typical" issues in a '94 that could be causing this? Any thoughts/suggestions/similar experiences are appreciated!

[Kb7tif]

About 1.5 months ago I bought my first Corvette and really jumped in the deep end.. a '94 ZR1 with a good number of miles (~140,000 km) that, I suspect, hasn't been treated overly gently at its previous owners'. I found when driving it home after purchase (~200 km) that after running for a while and warming up, the car develops what feels like a miss. It kind of makes a stuttering/gurgling/popping noise, the car jerks around a bit, and it keeps going. No Service Engine Soon light comes on. It doesn't hesitate when accelerating and seems to idle well enough. We just put in new spark plugs, hoping for an cheap/easy fix, but that doesn't seem to have helped the problem much (if at all). I've probably gone through 2.5 tanks of premium fuel since purchase, so it doesn't seem likely to me that it's just some bad or old gas.

I've read a lot of posts on the forum about a similar sounding problem in 90-92's that are making me wonder if it could be the fuel injectors on my '94, but I'm not sure if there are other "typical" issues in a '94 that could be causing this? Any thoughts/suggestions/similar experiences are appreciated!

You cannot go wrong with new injectors, and plug wires. Myself id leave the coils alone.

[XfireZ51]

Fuel pumps and filter(s)

[scottfab]

Just based on your description I'd start with
a common issue. It's the little hose running up to the
MAP sensor (at the read of the plenum). I can fill with oil.
That gurgling sound you describe sure sounds familiar to that.
If after clearing that next I'd check for any codes set.
Even if the SES (service engine soon) light is not on you may
get a leg up on things by looking there. Some codes if transient
don't set the SES light.
I assume you have a manual? If not we can help you along but
they are really invaluable.

If the stumbling kicks in when the ECM goes to "closed loop"
(warmed up) then there is more to do to isolate but let's see how the MAP hose check turns out.
Some things to tell us are:
1. Does it accelerate ok at WOT?
2. What's the fuel pressure?
3. Good solid spark on each cylinder when you pull each plug wire
out a few inches?
4. How do you feel about pulling the plenum if needed?

[Rusty]

Thanks for the suggestions! That gives me some suggestions to try out, which is great. Will hopefully get around to checking the hose to the map sensor this weekend and go from there. I'll post results as things go along, and may have more questions!

My shop manuals (purchased from eBay) just arrived on the weekend, so I can refer to those to help with these diagnostics and if anybody is able to point me towards a particularly helpful section/page #.

In response to scottfab's questions:
1. It accelerates well at WOT... doesn't seem to hesitate at all, and pulls... impressively, to say the least.
2. - 3. Will check the fuel pressure (and filter) and the spark on each cylinder on the weekend.
4. Personally, I'm not much of a mechanic (yet), but my Dad's been rebuilding his '86 with a new motor and many other TPIS things for the past few years, and he's helping me with the ZR1. So, between the two of us, pulling the plenum is just taking off a bunch of bolts. :-) Bring it on! I'll know more about ZR1 mechanics by the end of this, I'd imagine!

Thanks all for the ideas so far and I'll post more when I have some more information!

[scottfab]

................. pulling the plenum is just taking off a bunch of bolts. :-) Bring it on! I'll know more about ZR1 mechanics by the end of this, I'd imagine!

Thanks all for the ideas so far and I'll post more when I have some more information!

A few bolts yah, mostly.
Here is a link to a lot of help. There is more than one site (including this forum) to find How-to write-ups.
http://www.zr1netregistry.com/ZR1_howto.htm
It has a plenum pull section too.

[Rusty]

Thanks, scottfab. Looks like a great article and we'll definitely study it in detail before attempting the bolt removal (I jest and make light of what I fully expect to be pretty involved job... only joking, and I have great respect for those who have done this successfully and am appreciative of those who are willing to share the expertise they've gained).

With any luck it won't come to a plenum pull this time... would love to own the car for more than two months and go through more than 2 tanks of fuel before we have to do something like that!

[mike100]

The previous owner was trying to avoid pulling the plenum too I suspect.

Just resign yourself to the project- every single one of these cars will need injectors as they just don't last- either from age or alcohol blended fuels. If it was me personally, I would just buy new ones and change them (based on experience), but there is a good diagnostic to meter out all 16 injectors without taking anything apart. It might take about an hour to do all that.

The last time I had to work on the car, it really only took about 35 minutes to remove the plenum.

[scottfab]

Thanks, scottfab. Looks like a great article and we'll definitely study it in detail before attempting the bolt removal (I jest and make light of what I fully expect to be pretty involved job... only joking, and I have great respect for those who have done this successfully and am appreciative of those who are willing to share the expertise they've gained).

With any luck it won't come to a plenum pull this time... would love to own the car for more than two months and go through more than 2 tanks of fuel before we have to do something like that!


It's actually pretty painless. However, there are a few things to be careful of. Personally I got tired of replacing the gaskets. I mean you can gingerly remove it a few times and reuse but...... if they get damaged there you go. A bit more $ gone. I eventually just cut some out of black rubber in 1hr. Works great and NEVER sticks :mrgreen:
I reuse, reuse then reuse again.

Remember it's not if you have to get under there, It's when. Don't do it until you have new gaskets ready OR willing to cut some.

[Rusty]

Haven't yet had an opportunity to check the hose to the MAP, or much else, really (landscaping projects in the yard have been taking up my time), but I did find that the following codes come up:

DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #51: EEPROM Programming Error.
DTC #53: System Voltage Low.

Will hopefully have more time on the weekend to check things out.

[scottfab]

Haven't yet had an opportunity to check the hose to the MAP, or much else, really (landscaping projects in the yard have been taking up my time), but I did find that the following codes come up:

DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #51: EEPROM Programming Error.
DTC #53: System Voltage Low.

Will hopefully have more time on the weekend to check things out.

Great, now procedure dictates that you clear them and see what gets set again right away.

[XfireZ51]

Turn the ignition to ON, not start. Does SES BLINK once? I would check and make sure memcal was seated properly. Rough idle could indicate its running in Limp Home mode.

[Paul Workman]

I found when driving it home after purchase (~200 km) that after running for a while and warming up, the car develops what feels like a miss. It kind of makes a stuttering/gurgling/popping noise, the car jerks around a bit, and it keeps going.

This statement (in red) makes my compass point to electrical. "After it warms up"... is a classic winding kind of failure. Not saying this is your problem, but jerking and popping would go along with a cylinder or two, i.e., via the waste spark (two plugs of the same coil fire simultaneously in a loop)

But! Because you said it runs impressively at WOT, coils, wires, and plugs are set asside as culprits (for now - and you already eliminated plugs, you said). I say this because the dielectric constat (resistance to current flow) across the spark plug gap increases with increases in air/fuel mix - reaching its peak at WOT. (One reason top fuel dragsters use magnetos to throw vurtual lightning bolts at WOT!)

Relative to your symptoms, at least three important changes occur at WOT – which makes operation significantly different than normal cruising:
1) The secondary fuel pump turns on (i.e., if fuel starvation due to a failing primary pump was the problem before WOT, that problem - the stumbling at least - would be "resolved" when the secondary pump came on. Taping a fuel pressure gauge to the windshield and going for a drive will deny or confirm a fuel delivery issue: 45+ psi at cruise and about 52ish psi at WOT = not a delivery problem. I’d still change the inline fuel filter tho…

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/FPtestLarge.jpg

2) The secondary injectors come on so a marginal to failing primary injector would get some help from another injector coming to the rescue - to the point of possibly masking the symptoms at least, if not loss of power. Fuel starvation in a single cylinder is serious – leading to burned valves, for example. And, unlike your daddy’s 86, pulling the heads on the LT5 is a major operation involving pulling the entire engine and re-timing the cams, etc, etc. I’d wanna eliminate fuel delivery first, before putting any more miles on ‘er.
3) Didn't you mention TPS, and ECM errors? With those kind of miles, the contact point between the wiper and the rheostat inside the TPS could by now be carbon etched or pitted. This would lead to an erratic "off-on" voltage signal to the ECM which would really confuse it. However, at WOT the contact point between the wiper and the rheostat is in a "clean" place where the wiper doesn't normally hang out constantly, so the signal is steady and the ECM is not getting mixed messages.

Yeah, "more potatoes for the stew pot"… The FSM will help a lot - at least up to the point of requiring a Tech-I to manipulate something. IMO, the danger with these "I had that problem and I did this to fix it" threads is some start changing parts till the problem goes away (aka "shotgunning"), in place of methodical troubleshooting. I don't get the feeling you are that type. Whe asking for ideas, I take these scattered tidbits as reference "bookmarks" to be considered in due course rather than indication of stuff to change out.

One premier exception to "shotgunning" might be the OEM injectors - especially in a 90-92 which were affected by alcohol in the fuel. But, even a 93-95 at 140k miles and 17+ years, they can have issues. Yeah, I'd want to eliminate the other stuff first, but keep changing them in the mix.

BTW!! Welcome to the BROTHERHOOD OF THE BEAST! This issue of yours should be realtively simple to fix (if not simple to find). After you get this item fixed, I have a feeling your dad is gonna want to "test drive" your Z a lot, just to make sure it is running OK.;)

P.