Weird stuttering issue

[Mikey]

Situation:
Bought a '91 368 Corey Henderson built Z with 25K miles a few months ago. Drove it home from WI to FL. Behaved the whole way.
Once in FL I had opportunity to do a WOT run to 7100 rpm in 2nd gear. It hard stumbled (coughed) once for a split second around 3200rpm but otherwise pulled strong. Repeatable event.
Moved from FL to TN drove the car up and behaved all the way.
This morning it was 38 degrees. Leaving neighborhood in 1st gear 20-30% throttle the engine exhibited a 2hz stutter as if I was on/off throttle in the 3000-3500 rpm range. About 6 or 7 "coughs" in a row. Cleared up above 3500 rpm. Was able to repeat it immediately after but no more. Car seemed fine afterwards except for the single cough/shudder that is fairly repeatable as described above.
Seems only to do it in 1st or 2nd. Not 3rd or 4th.

I've put 3,000 miles on it with no other issues.

Thoughts?


Update:
Had read a story some months back about early LT5 engine failures and the discovery of a female at the plant who cold started every ZR-1 with her foot on the floor. The article didn't say anything about a cold engine rev limiter but I suspected maybe the ecu had one implemented after engineering witnessed this crazy abuse.

I had lunch with the assistant chief engineer for the C8 LT2 engine in April (I grew up with him, friends since 5yo) and neither he or his boss knew for sure about possible 2nd rev limiter back in that era. And of course my car was too warm still after lunch to replicate the cold performance symptoms.

Watch this video. At 13:30 he discusses the story I had read about and confirms indeed a second rev limiter was implemented!

https://www.youtube.com/watch?v=6Vs2tdR-9C8&t=810s

[Corvette95]

Fuel pump going out, where do you keep your level of gas or what was level when this happened ? Spark plug wire breaking down? Fuel injector going out or fuel regulator? Check your MAP plug, you need a scanner.

[wfot]

In my understanding, the ECM will "protect" the engine if certain conditions are met/not met. 2 of these are;

engine service light = on
secondary map sensor signal is not within range - it is special MAP sensor that is under the ECM tray on your's - it is different than the one attached to the back of your intake - they look the same

if any of these are triggered AND the secondaries are activated. the ECM will CUT spark/Fuel at anything above 3200ish RPM. I am assuming there are no codes/service engine light

in other words; drive it in conditions were the secondaries are inactive - you will not ever feel the shudder / mash the peddle/active the secondaries and the studder will show up. this is protect theyself design.

I had a bad studder with no codes - it was broken wires to the secondary MAP sensor and was intermittent - took 4 months to figure it out.

just a possibility from my seat in the peanut gallery

and keep in mind, you will most likely have a modified chip, that may alter many parameters - including "please ignore the secondary MAP sensor code alert"

I would call corey and start asking questions there as following the FMS may not account for any chip alterations...

John

[Corvette95]

Make sure your PROM is seated and locked in place as well.

[Paul Workman]

FUEL...AIR...SPARK!

I'm assuming Cory put alcohol tolerant injectors in it when he did the 368 upgrade, so (for now) I'd want to verify dynamic fuel pressure FUEL PRESSURE first: Harbor Freight can supply a fuel pressure gauge and an extension hose so that you can tap into the fuel rail test port (front end of the passenger side rail). Then you can tape the gauge to the windshield and go for a drive.

Cruising at around 50-60 ish mph the pressure will be ~ mid 40s (mine hovered around 45-46 psi @ 45-50 mph).

Slapping the throttle wide open and the pressure leaps to the 53-55+ psi range.

If you pressure(s) are low, especially on the WOT portion of the test, starvation is the issue, and you want to address that ASAP to avoid serious issues resulting from a lean situation...ask me how I know!

I wanna say, one of the best damn tools I ever bought for my Z was a scanner capable of OBD-I reading!! Having one will really cut to the chase! I'll have to defer to others Marc Haibeck, or Dominic or Pete or -=Jeff=- for a recommendation for a suggestion (as my AutoXRay unit is no longer available).

OK, fuel pressure OK, and no scanner, spark plug condition comes to mind. By reading the plugs, they can often give clues as to one or ALL cylinders operating condition.

Again, here's where a scanner pays for itself, look at the O2s: checking for uniformity in operation, and cross counts especially, and they should be in the hundreds. (An O2 with low cross counts indicates a lazy O3 sensor. For verification, you can swap them side to side and re-test. If the one lazy O2 is still reading low cross counts, then you have located at least ONE potential issue.

A NOID LIGHT will let you know if you have a cylinder with a weak spark, along with the plug being wet or sooty - confirming what a noid light is suggesting.

Coils: (see attached) Waste spark = two plugs firing simultaneously as each coil is attached to two plugs in series:

1&6, 4&7, 2&3, 5&6.

Resistance (AC/Delco) between spark plug (secondary) terminals ~ 5.2kΩ, but resistance between (primary slots) only ~ 1.5Ω. Resistance between primary and secondary is infinity (or better be!).

Note: A common issue is for the secondary side to be open. If that happens, the spark may jump the break internally, but the resistance (air gap) will impede the current to the plugs resulting in a weak spark. Replace one, replace them ALL! ( If one fails, chances are they are at the end of their life expectancy and it is safer to change them all - take a chance on the new electronics "infant mortality" - and not have to troubleshoot each of them again as they fail. Just a thought. (Folks have had good luck with MSD products - or they used to before they were made in China).

It all this checks out and the issue remains, we can take further steps...

[Mikey]

Thanks guys. A little more info:
No CEL lit. Will have to check if there are any codes when I can get a scanner.
Was told Corey disabled the secondaries. Car is always in "full power" mode.
Fuel level at 50%.
Regarding O2 sensors I had just left the house so wouldn't it still have been in open loop and ignoring O2?

Understand fuel/air/spark. From experience this seems most likely to be a sensor issue like TPS or MAP perhaps but for sure I will check out fuel pressure and also spark plug conditions. I also have an ignition scan tool I can read strength at the wires or coils so I will try to do that. Unfortunately much of my stuff is still in boxes from the move and maybe a year from getting my garage situation sorted.

Thanks all for the thoughts below and Paul especially for the target numbers to look for during diagnostics.

Cheers,
Mike

[Mikey]

Ok I have to shamefully admit I have not made time to further diagnose yet but do have some new findings to share.

Had not driven it since this started 2 weeks ago. Just needed to pull it out of garage today to move some things around. Decided I would let it idle for a bit. But after first pulling it out and while engine was still cold I decided to stab the throttle while out of gear. Engine cutout at 3400rpm like a rev limiter. So then I applied light throttle and as the rpm's increased, at 3400 the engine acted again like a soft rev limiter and with light throttle still the engine "hunted" between 3400 and 3100. It would go up and down in a sinusoidal kind of way.

I let the car idle for a bit until water temp was 85C and tried again - this time no issues and no rev limiter symptoms.

Do any aftermarket tuners enable a soft limiter until the car is warmed up?

[jss06c6]

Originally Posted by Mikey

Ok I have to shamefully admit I have not made time to further diagnose yet but do have some new findings to share.

Had not driven it since this started 2 weeks ago. Just needed to pull it out of garage today to move some things around. Decided I would let it idle for a bit. But after first pulling it out and while engine was still cold I decided to stab the throttle while out of gear. Engine cutout at 3400rpm like a rev limiter. So then I applied light throttle and as the rpm's increased, at 3400 the engine acted again like a soft rev limiter and with light throttle still the engine "hunted" between 3400 and 3100. It would go up and down in a sinusoidal kind of way.

I let the car idle for a bit until water temp was 85C and tried again - this time no issues and no rev limiter symptoms.

Do any aftermarket tuners enable a soft limiter until the car is warmed up?

Full power is not engaged until car is warmed up and in closed loop.

Sent from my SM-G950U using ZR-1 Net Registry mobile app

[Mikey]

The recently released video about the C4 ZR-1 that had Hagerty?s name at the bottom may have solved this. The speaker revealed that a second rev limiter was programmed in for cold starts all because a particular quality control female employee was starting up the brand new cars with her foot depressing the throttle 100%. This led to an engine failure or two during and engineering, after discovering the root contributor, decided to put in the second rev limiter. Hear about it from 13:00-14:15 in the Hagerty video. Given that my car is only exhibiting the behavior described in my first post until the engine has slightly warmed up I believe I am experiencing this described second rev limiter. I learned something new today. In general, this second type of rev limiter was not really implemented on main stream cars until about 20 years later!

[32valvZ]

I dunno, but with the 80 plus vehicles that have gone through my hands, I would never drive them hard until mostly or fully warmed up. I suppose Im old school, but and engine, is an engine, is an engine.... regardless of modern computer aided fuel systems making the engine "seem" like its ready to be driven under any circumstances, the engine is still made of metals which need some time to warm up before flogging... Oil needs a chance to warm up as well as coolant.... Like I said, maybe Im just old school, ya, I am... LOL.... but none the less, letting my autos and trucks warm up, I believe is good practice.