I realize that the SD isn’t a car site, but I am having a problem and I am having no luck getting answers on some of the Nissan boards, so I thought I would try here.
I have a 1988 Nissan 300zx. The car is running very well and smooth until you push the throttle to the floor. Under full throttle the car loses all power like someone threw an off switch. I have tried everything I can think of to diagnose the issue and I’m coming up short. What I have done so far:
Replaced the fuel filter and fuel line from the filter to the fuel rail.
Replaced every vacuum line
Put a guage on the fuel line to read fuel pressure under load. The pressure stayed high, so I don’t think that it is a fuel pump problem.
Now, to make everything stranger, the problem solved itself a few weeks ago and then reappeared. I put in a new temp. sensor and when I restarted the car and test drove it the problem had completely resolved. Fast forward a few weeks and I take the car out for a drive, nothing has changed, and the problem is back. I was thinking it might be timing, but timing wouldn’t fix itself and then unfix itself, so it can’t be that.
Digging around the factory Nissan manual I saw something in there that said to check the flexible tubing that runs from the airbox to the AFM sensor for leaks. My son pulled the intake tube off tonight and there is a definite tear in the tubung, right where the clamp holds it onto the tube from the airbox. The tear is about 3 inches long. Could this possibly be the problem? If so can it be repaired or is the only solution to replace the part? I’m thinking of using something like silicone sealant to seal the tear. I’m not sure if that would be sufficient to seal the rubber up. The part that is torn is not under any stress, it fits into another lenght of tubing and has a clamp sitting on top of it.
The main thing is I need help narrowing down what could be the cause of the power loss under acceleration. FWIW the engine doesn’t exhibit the same issue when sitting still and the throttle is floored. I’m usually pretty good at figuring out engine problems, but this one has me stumped!:smack:
Your intermittent problem is most likely electrical. On OBD-1 systems one of the first code reader checks is a wire wiggle check.
I would run a couple of strings trough into engine and have a helper wiggle the wirering harness in a couple of spots when the power loss is present. And the wireing near the temp sensor would be 1st checked.
Some of the OBD-1 systems sensors were, well for lac of a better description, double acting, in that some temp sensors would give a value up to a point and then start over, so the problem could certainly be in the ECM also. Very hard to trouble shoot.
Good luck and i hope you get better replies than this.
This could be electrical, or vacuum-based, depending on your interpretation of “cuts off like a switch”.
If it truly cuts the spark, or the power from a sensor to the computer, that’s one problem.
If it revs until a certain point, then cuts off, only to “catch” and run again at lower RPM, that’s different.
You’ll first need to determine which is happening. It will be easy with a buddy watching a spark tester while you give it gas, or you could do it yourself. If you’re getting spark when it dies, replace that tubing part. Don’t play games with silicon. You may or may not fix it, and you won’t know for sure. It’s there for a reason; just replace it.
If no spark at max rpm, then the electrical wiggle test is next. If no luck there, check all the sensors for proper operation. A Haynes or similar manual will give you the specs you need.
Don’t forget to clean the MAF sensor, check the CPS, and all vacuum hoses. Use a carb spray or similar on the hoses. You can have a hole that you don’t see, but it will react when you spray something on it. The revs will change when you spray a hole.
Turbo, or not? (That might help a better mechanic than me give you a better answer.)
Either way, that air leak sounds like an easy enough fix. Whatever you use to fix it, you’re gonna want to make sure that nothing is going to come loose and get sucked up. IMO, you’re better off just scouring the junkyards and/or ebay for a replacement. As for it or the MAF sensor being the culprit, at WOT (Wide Open Throttle) the ECU would be in “open-loop mode” and be completely disregarding that (and other) sensors, anyway.
As has been pointed out, I’d start looking at ignition. First pull the spark plugs, since you can diagnose a lot of problems by reading spark plugs. And unless they’re relatively new and in good condition, you might as well replace 'em now that you’ve got 'em out. How about the plug wires? How old are they and what condition is their insulation in? If their exteriors are brittle and/or cracked at all, or if they’re ancient, might as well replace them, as well. After those, consider inspecting/replacing the distributor cap and rotor, if necessary. And hope it ain’t the distributor itself.
IIRC an 88 300 has an air flow meter NOT a mass air flow sensor. Totally different animal.
In no particular order here is what I would check
Replace the hose don’t dick with trying to patch it, it won’t work. Make sure there are no tears between the ASfM and the engine.
You checked the fuel pressure good. Does the FP regulator have a vacuum line to it? if so did the pressure rise when throttle is opened?
Speaking of which pull the vac hose off the regulator and smell it. Smell gas? Leaky regulator.
When the engine falls flat, does it die or just lose power?
Does the tach drop like a stone when the engine has issues? If so look at ignition primary.
Where does the ignition get its doped signal? Pickup in the distributor or a crank pickup?
Take the hoses off the AFM. Look inside see the little door that moves? Are the sides of the doorway scratched or damaged? Does the door move smoothly? If equipped with a poppet is it in good working order? If there are scratches, sticky spots or a blown poppet valve you need a new AFM for sure.
How old are the plugs, wires, cap and rotor?
Good luck.
If the intake air duct between the airflow meter and the throttle body is torn, replace it. With vigorous acceleration under load (in gear) the engine will torque over on its mounts and pull on that duct. This is what caused it to tear, and this is what causes the slit to open when you stomp on it while driving. The engine sucks in unmetered air, which means the computer doesn’t know to let in the corresponding amount of fuel. Result? It falls flat on its face.
Sometimes those ducts can be patched temporarily with duct tape, depending on just how the tear is situated. The patch has to be able to seal the leak and hold against the pull under aceleration. Forget any kind of glue – it will be ripped apart in short order.
I missed this last night.
Not only wrong, but one of the more unique theory’s of how an ECM works ie ignore all the sensors and use your internal crystal ball for timing and fuel metering.
For the record the cars stays in closed loop at WOT ( or WFO if you prefer). The ECM momentarily stops responding to the oxygen sensor. As soon as you lift your foot lambda control resumes.
