Stumbling Issue
 

I ran this by Marc but I don't want to keep monopolizing his time.
[I've got a '94 ZR-1 that recently started acting up. When I first bought it about 5 years ago it had some stumbling and stalling issues that were sorted out by replacing the spark plugs. Since then it's run wonderfully until very recently. About thirty minutes into a drive in temperate conditions, (maybe 60 degrees out) it suddenly started exhibiting very disconcerting, lurching losses of power. All gauges were normal. No engine lights. I couldn't figure out exactly what would cause the lurches. It would happen when I accelerated from low rpms at highway speed. It would happen at high rpms. I tried leaving the car in uncomfortably low gears at highway speed, keeping the rpms relatively high. I tried just cruising at a normal rpm with no acceleration and the stumble would happen then too. The occasional and seemingly unpredictable power losses would still happen no matter the driving condition. It felt like half a stall and very frightening at highway speed. I tried power key both on and off. It happened in both conditions. I've felt something similar briefly once before when I'm pretty sure the A/C system was malfunctioning and the belt was dragging (saw smoke that time and it was clearly associated with turning the a/c on. It never happened like that again.) This same power loss happened with a/c on and with the a/c system off. It also started stalling when idling. It's never had this problem before.]

After I emailed Marc the above in brackets, I took a look under the hood again and realized the airduct had slipped off by just a little bit. Putting it back in place and tightening it fixed almost all the issues except that engine would still stumble if I tried to accelerate from low RPMs (often right about 1750rpms in first or second.) It idled fine. The stumbling wasn't nearly as dramatic and the car was driveable.

I figured the air duct had become weak and was collapsing so I replaced it with a silicone one from Jerry's Gaskets. Drove it for thirty minutes and she was an absolute monster. Didn't seem to matter how I put the gas down she pulled hard. I parked it for about 10 minutes, got back in, and the stumbling returned. It seems to happen reliably around 1500-2000 RPMs primarily when accelerating in first or second. It's a sudden dramatic loss of power which can usually be overcome by giving it a TON of gas. It's driveable, but unnerving.

Any thoughts? Marc suggested the MAP sensor before I discovered the air duct issue. Thanks in advance to this incredible community.

Re: Stumbling Issue
 

Possibly a coil on its way out.

Re: Stumbling Issue
 

Check your fuel pressure, pumps, and hopefully your injectors are okay.

This sounds a lot like my newly acquired 58k mile 90. I believe I have it traced down to a faulty fuel pump, which is good, because they're inexpensive, and an easy job. But I'll know when the car comes back from paint.

Here is some information for you to start with.

http://www.zr1specialist.com/HAT%20W...in%20Place.pdf

http://www.zr1specialist.com/HAT%20W...eplacement.pdf

http://www.zr1specialist.com/HAT%20W...20Shooting.pdf

Re: Stumbling Issue
 

Fuel filter? Pumps? Pump screens?

Re: Stumbling Issue
 

Along with the other items, check the 02s. If it runs great before it warms up, once the engine warms it switches from preset values to adjusting based upon the 02s feedback. When you get hard into the throttle, it goes back to the preset values.... scan your 02s to make sure they are functioning properly.

Re: Stumbling Issue
 

Thanks everyone. This is super helpful! I'll try to post back when I get the issue hunted down.

Re: Stumbling Issue
 

Try the coolant temp sender , if its not working correctly , it wont send the correct reading to the ecm . And put a gm part back in ! some aftermarket parts just dont meet the grade !

Re: Stumbling Issue
 

I'm on board with coils

Re: Stumbling Issue
 

One of the best investments you can make for ur ZR is datalogging equipment. Relatively inexpensive, and less expensive than throwing parts at the problem and wasting time.

Re: Stumbling Issue
 

Yeah but what can we use is there anything newer on the market?
I don't know anything about TunerPro
Id love to have real time data