Backfiring at WOT: 280z

[RT Hunter]

What works:

- Idles smooth

- If gradually accelerating, the engine runs smooth throughout the entire power-band with no issues

Problem:

- During idle, if pressing gas pedal all the way down suddenly, it will rev up to about 3500rpm, backfires and surges, and the rpm will drop to about 2500rpm and rev back up at 3500rpm and keep repeating as long as it's at WOT.

Does this sound familiar to anyone?

[Wade Nelson]

I'd check for a plugged exhaust system / cat converter. Pull the 02 sensor out, let it roar, take it out and drive it.

[Zed Head]

Could also be the TPS. Read the Engine Fuel chapter, it describes how the ECU delivers "full enrichment" above a certain throttle opening based on the signal from the TPS. Shows how to test it also.

No throttle signal to the ECU, no enrichment, lean mixture, popping and surging. Maybe.

[Captain Obvious]

- If gradually accelerating, the engine runs smooth throughout the entire power-band with no issues

Problem:

- During idle, if pressing gas pedal all the way down suddenly, it will rev up to about 3500rpm, backfires and surges, and the rpm will drop to about 2500rpm and rev back up at 3500rpm and keep repeating as long as it's at WOT.

Let me make sure I understand what you're saying... Are you saying that you put your foot to the floor when you are driving around, and it's fine. But you put your foot to the floor when it's sitting in neutral with no load and it backfires and surges?

I was thinking the same as Zed... Have you checked that your throttle position switch is doing what it's supposed to?

Wade, FYI... no cat in 77 (except CA?), and no O2 sensor in any 280Z. That didn't start until a few years into the ZX.

[RT Hunter]

It happens even if crusing around and i put the pedal to the floor, it will pop and surge around 3500rpm. I have no problem driving it fast and such, but only when i floor it all the way.

I will check out what you guys mentioned. Thanks!

[Wade Nelson]

I'd put a vacuum gauge on it right away. Take you 30 seconds. That would SHOW you any breathing problems, valve issues, etc.

http://www.secondchancegarage.com/public/186.cfm

[RT Hunter]

I'll check the vacuum first thing when I get a chance. Thanks.

Just to add to the information, if I am idling, and then super instantly slam on the gas pedal to the floor, it will almost stall out for a second, but will the rev up. I think this issue and the one in the original post is related to each other somehow. It feels as if gas can't keep up with the sudden WOTs (but gradual WOT will work smoothly up to redline).

[Captain Obvious]

It happens even if crusing around and i put the pedal to the floor, it will pop and surge around 3500rpm. I have no problem driving it fast and such, but only when i floor it all the way.

Yeah, methinks you're running lean at WOT for some reason. There are lots of things that can cause that, from the TPS adjustment to a bad FPR or fuel pump, etc...

Have you downloaded the FSM and the EFI troubleshooting guide yet?

[FastWoman]

Yeah, methinks you're running lean at WOT for some reason. There are lots of things that can cause that, from the TPS adjustment to a bad FPR or fuel pump, etc...

Have you downloaded the FSM and the EFI troubleshooting guide yet?

Methinks the same thing, and there can indeed be many causes. I suspect the most common cause is a big vacuum leak (or many smaller leaks). I've coined the term "yogurt cup test," which you can search. It will allow you to verify whether your intake is tight -- all except for one item, which is the big rubber boot between the AFM and the throttle body. (You'll have to inspect that manually.)

There's also an apparent issue common in the 1978 ECU, and I suspect also common in some 1977's as well (similar year). Our ECUs are open-loop (not self calibrating) and are of an analog design. Over the decades, in rather harsh conditions, the semiconductors drift in their responsiveness, and the predominant direction of drift seems to be towards shorter pulse widths, hence less fuel delivery and leaner. There are a few different fixes and theories to go with them. However, before you modify your ECU to deliver more fuel, you should first determine whether you have fixable problems that are causing you to run lean. That means going over your system with a fine-tooth comb, checking and verifying every single component. If all that is left is error in the ECU, that can be remedied. But first you must get there through a process of elimination!

[Wade Nelson]

if I am idling, and then super instantly slam on the gas pedal to the floor, it will almost stall out for a second, but will the rev up. .

Sounds EVEN MORE like a plugged exhaust system. Loosen some bolts at the down tube so it's got an intentional leak and go drive it.

[LeonV]

Exhaust systems don't just get "plugged". TPS is my first thought, vac leaks seem unlikely from his description, a stock EFI Z won't idle smoothly at all if there's a leak large enough to hinder WOT operation that much.

RT, have you washed the engine bay lately or been in a downpour? Water can screw with the TPS, even if it's adjusted correctly. Follow Captain's advice and download the FSM and EFI guide.

[RT Hunter]

Yeah, methinks you're running lean at WOT for some reason. There are lots of things that can cause that, from the TPS adjustment to a bad FPR or fuel pump, etc...

Have you downloaded the FSM and the EFI troubleshooting guide yet?

Yeah, I got the FSM. I will try those series of troubleshoot tests when I get a chance. Thanks!

Sounds EVEN MORE like a plugged exhaust system. Loosen some bolts at the down tube so it's got an intentional leak and go drive it.

I actually have a leak in my exhaust (possibly at the header), and I see a slight gap at my down tube. I can smell exhaust fumes from my engine bay, and my eyes stings if I stand there too long. Clothes will soak with exhaust odor within 10 minutes of being around the engine bay.

Also, another issue that might be related is that I lose fuel pressure after the car sits. If I let it sit for 1 or 2 days, it takes about 4 cranks to get it started. It stumbles really bad when it's trying to crank up, but once it cranks up, it idles fine. The car will startup instantly after I turn off the car and back on.

I usually get time on the weekends to mess with the car, so I'll keep everyone updated on my tests and findings then.

List of todo:

- Vacuum check

- TPS and other electrical component checks via FSM

- Fuel pressure check