Piston Slap: Too much talk just for vapor lock?
(The following email conversation to pistonslap@hagerty.com has been gently edited for clarity and length. If you like what you see, you should also email me because I’m starting to run low on questions! – SM)
Dave writes:
Hi, I own a ’76 Datsun 280 Z I bought new and it is still stock. I didn’t drive it for a few years and to my surprise started right up. However, after a few weeks of driving it starts to bog and hesitate around 2000 rpm ONLY after it warms up. It runs great cold.
When it starts to bog the fuel pressure drops and jitters between 20 & 25 psi instead of the factory 36 psi. It seems like when the engine warms up the fuel pump may not be receiving the full 12 volts of DC current, hence the lower and jittering fuel pump pressure. I’m thinking it might be the fuel pump voltage relay or maybe a leaking fuel injector.
I cleaned all fuel lines, replaced the fuel filter 3X and replaced the fuel pump with 2 different new ones, and replaced the sensors in the thermostat housing. I also replaced the fuel pressure regulator, drained the fuel tank (the fuel coming out was clear), and filled it with ethanol-free premium. I also replaced the spark plugs (old ones looked fine) and plug wires. Any thoughts?
Sajeev replies:
Thanks Dave, you’ve done a great job assessing the situation. I have concerns that either the battery or the alternator aren’t doing their jobs anymore. Or maybe the fuel pump wiring has gone bad, and likely needs to be re-wired (preferably with a relay and a fresh power wire directly from the battery). Can you tell me about the condition of your alternator, the age of your battery, and how your fuel pump was wired?
Dave replies:
The battery and alternator are both new and the voltage across the battery is around 13.6 volts of DC current when the engine is idling at start-up and at operating temperature.
There is a relay for the fuel pump. It doesn’t seem like the fuel pump relay is bad because it runs perfectly for the first few minutes after start-up. My question is why the fuel pump delivers 36 psi when cold but only 20 psi when warm? (Factory specs for the fuel pressure is 36 psi.)
Sajeev replies:
Yes, the relay suggests that heat/resistance as the car warms up wouldn’t be an issue. But any vehicle this old can have bad wiring, so the problem you’re experiencing could be from a voltage drop. Have a look at this:
Dave replies:
Hi Sajeev, for the fuel pump voltage I was thinking of just hooking a DC voltmeter to the pump terminals in the back of the car. I would note the difference in voltage (if any) when the engine was cold and then hot while bogging. Let me know what you think about this.
Sajeev concludes:
I think your cold and hot voltage testing at the fuel pump is a great idea; it’s easier than what I suggested and accomplishes the same result.
But I fear I didn’t consider other potential problems. Like vapor lock, which includes several items to check, including the (slim?) possibility that a clogged EVAP system is a source of the problem. Considering the age of the vehicle (and the emissions-reducing technology available in the Malaise Era!) maybe it’s the EVAP system, not the voltage to the pump.
Apparently, your EVAP canister has a filter (above) that could be clogged. And having a clogged filter means there’s a (slim?) chance that fuel system performance deteriorates as the system heats up. No matter, you should test the aforementioned voltage and then see what you can do to resolve vapor lock. What say you, Hagerty Community?
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I’m a pretty old-school type of shade tree mechanic, but to me, fuel pressure issues when an engine warms up suggests vapor lock as first on the culprit list.
Check the Fuel Injection Coolant Temperature Sender. Or better yet just replace it with a factory replacement That’s a classic symptom and the mixture is highly dependent on it. People tend to overthink AFM controlled systems when they’re pretty straight forward. Don’t sweat the fuel pressure or the relay contacts, flux capacitor or any of that until you have checked the timing, sensors etc. step by step. If it tests good move on. Don’t tamper with the Air Flow Meter.
I neglected to mention in the previous post. One of the things they taught in Bosch Jetronic school was that; If it was a Bosch part, move on, If it’s not a Bosch part; Suspect it.
There is a whole range of temperature sensors, literally hundreds of different ones, so it’s critical to have the corrrect one because it goes to the control unit which controls the amount of fuel dispersed by the injectors based on how much air the AFM is reading. Check the injector seals, vacuum leaks and less likely a leaking cold start valve. A leaking cold start valve could explain the fuel pressure drop, but typically it would also start hard and affect the idle more. Didn’t mean to turn it into a tutorial, but I know how frustrating these old systems can be.
Nothing wrong with “a tutorial” from a knowledgeable source, Steven! But, I’m missing your “previous post”…?
Check the Fuel Injection Coolant Temperature Sender. Or better yet just replace it with a factory replacement That’s a classic symptom and the mixture is highly dependent on it. People tend to overthink AFM controlled systems when they’re pretty straight forward. Don’t sweat the fuel pressure or the relay contacts, flux capacitor or any of that until you have checked the timing, sensors etc. step by step. If it tests good move on. Don’t tamper with the Air Flow Meter.
Coolant sensor is reading wrong.
Check the Fuel Injection Coolant Temperature Sender. Or better yet just replace it with a factory replacement That’s a classic symptom and the mixture is highly dependent on it. People tend to overthink AFM controlled systems when they’re pretty straight forward. Don’t sweat the fuel pressure or the relay contacts, flux capacitor or any of that until you have checked the timing, sensors etc. step by step. If it tests good move on. Don’t tamper with the Air Flow Meter. Must be a keyword issue, says awaiting moderation every time
Sorry, it said waiting for moderation. Heere it is:
Check the Fuel Injection Coolant Temperature Sender. Or better yet just replace it with a factory replacement That’s a classic symptom and the mixture is highly dependent on it. People tend to overthink AFM controlled systems when they’re pretty straight forward. Don’t sweat the fuel pressure or the relay contacts, flux capacitor or any of that until you have checked the timing, sensors etc. step by step. If it tests good move on. Don’t tamper with the Air Flow Meter.
It’s awaiting moderation. Musts be keyword related.
The description of the problem is classic of a coolant sensor reading afoul or having a bad winding.
Check the Fuel Injection Coolant Temperature Sender. Or better yet just replace it with a factory replacement That’s a classic symptom and the mixture is highly dependent on it. People tend to overthink AFM controlled systems when they’re pretty straight forward. Don’t sweat the fuel pressure or the relay contacts, flux capacitor or any of that until you have checked the timing, sensors etc. step by step. If it tests good move on. Don’t tamper with the Air Flow Meter.
Must be a keyword issue, says awaiting moderation every time
When I saw the fuel pressures referenced in the post, I did some googling because they sound like EFI pressures for a car that was in my mind carbureted
Mr. Google quickly pointed out my error and informed me that the Z cars were injected since 1975
Mr. Google also told me that the fuel pressure regulator in that system is controlled by engine vacuum and fuel pressure will vary with changes in vacuum. Your changes in fuel pressure may be more of a symptom than a cause
Your car is running lean for some reason – I would look for vacuum leaks, failed temperature sensor (as suggested above), O2 sensors, etc
They also changed their firing order in 76 or 77 founf it in Mitchell at the time. 2 wire swap. O2 sensor could be ruled out.
If it stalls @ 2000 rpm under load and
suddenly picks up and goes, it’s the CTS. Very common symptom. It usually won’t do it without a load on it. Vacuum leaks are a possibility but they cause excessively high idle when cold. Anything rubber should be thoroughly checked when a car has sat. Dried out vacuum lines and seals can cause a lot of grief.
IIRC, this is a common problem with these cars. Perhaps insulating the fuel lines (at least) near heat sources and in the engine compartment will prove beneficial.
Good point, I think you are on to something.
I can’t add much but what I can say will be brief: it’s not vapor lock. Pressurized systems aren’t vulnerable.
Aren’t as vulnerable.
That’s correct. True vapor lock isn’t very common unless it’s grossly out of time or the camshaft is misaligned. We used to see it a lot in the mid 80’s carbureted cars with Califiornia emissons and high altitude compensation, or on super hot days. Usually ended up as an empty float bowl due to vacuum compensators or float adjustment.
…but but it did make for a catchy column title!
Isn’t click bait the most important thing nowadays? 😁