Rough running when hot

General information about the first-generation Nissan Maxima in the US. What was the Datsun 810 became the luxury leader Maxima in the US in 1981.

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Lucas
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#16

Post by Lucas » 13 years ago

83_maxima wrote:I'm bumping this thread because I think I have a glorified version of this problem that seems to have gotten worse as of late.

When I drive the car for long periods (read: up to operating temp for several minutes) shut it off, then try to restart it a short time later (while it is still warm) it starts and dies - immediately. This does not happen if allowed to cool down. I have to hold the pedal down and it will chug, chug, chug and eventually it will rev up. I need to rev it pretty hard - 4-5K to keep it running, or very quickly get it in gear and go. Under load the issue will go away. It feels like once the fuel is used or something eliminated in the lines (air?), it is fine. Runs perfectly.

Feels like it is loading up with fuel maybe? Bad cold start or thermotime perhaps?

Unlike Kassim's issue, it will not go away if I keep the key in the on position for a short while before trying to start it. The thermotime and water sensors are relatively new, but I haven't checked them as being OK yet.

I thought I'd probe here for some suggestions while I check the sensors' reistance and see what I have.

TIA.

Your problem sounds like it could be a leaking fuel injector(s). A car I used to own (not a Maxima) did the same thing, with the hard starts after warm and the chug, after a mechanic I went to replaced the fuel injectors. I did not realize this until after I no longer owned the car.

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83_maxima
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#17

Post by 83_maxima » 13 years ago

Lucas wrote:
83_maxima wrote:I'm bumping this thread because I think I have a glorified version of this problem that seems to have gotten worse as of late.

When I drive the car for long periods (read: up to operating temp for several minutes) shut it off, then try to restart it a short time later (while it is still warm) it starts and dies - immediately. This does not happen if allowed to cool down. I have to hold the pedal down and it will chug, chug, chug and eventually it will rev up. I need to rev it pretty hard - 4-5K to keep it running, or very quickly get it in gear and go. Under load the issue will go away. It feels like once the fuel is used or something eliminated in the lines (air?), it is fine. Runs perfectly.

Feels like it is loading up with fuel maybe? Bad cold start or thermotime perhaps?

Unlike Kassim's issue, it will not go away if I keep the key in the on position for a short while before trying to start it. The thermotime and water sensors are relatively new, but I haven't checked them as being OK yet.

I thought I'd probe here for some suggestions while I check the sensors' reistance and see what I have.

TIA.

Your problem sounds like it could be a leaking fuel injector(s). A car I used to own (not a Maxima) did the same thing, with the hard starts after warm and the chug, after a mechanic I went to replaced the fuel injectors. I did not realize this until after I no longer owned the car.
All of the injectors are GOOD. All rebuilt/flow/leak tested the middle of last year.

But I think you may be onto something with the leak in the fuel system. I am leaning toward the lines. When I get some time this week I am going to put new flexible lines on it and see what I've got.

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83_maxima
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#18

Post by 83_maxima » 13 years ago

kassim503 wrote:Show us more pictures of your grille badge!

From what I see I like it
The badge is from a late 70s 720 I found in the JY. I cleaned and painted the badge, then notched it for use with the max grill. Used stock mounting hardware from the 720.

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philip
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Re: Rough running when hot

#19

Post by philip » 13 years ago

kassim503 wrote:My gasser runs rough for the first several seconds when the engine is started at operating temperature (180-190F) It seems to get worse if the engine is hotter and it dosent have this problem if the key is put in the ON postion for a minute or so before cranking the starter.
With winter comes gasoline reformulation having a lower flash point. This is accomplished (typically) by increasing the oxygenate additive. This is fine in cold weather but makes the engine more vulnerable to old fashioned vapor-lock. After a short heat soak, restarting the engine can be difficult until the vaporized fuel ahead of the injector(s) is replaced with solid liquid fuel.

Avoid Premium fuels (higher oxygenate), avoid alcohol enriched fuels, try a different brand of low octane fuel, avoid prolonged idling, avoid shutting down very shortly after driving at freeway speeds.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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asavage
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Re: Rough running when hot

#20

Post by asavage » 13 years ago

philip wrote:. . . more vulnerable to old fashioned vapor-lock. After a short heat soak, restarting the engine can be difficult until the vaporized fuel ahead of the injector(s) is replaced with solid liquid fuel.
Let's get our terms straight: vapour lock is when the fuel pump lowers pressure on the inlet and the hot fuel changes state from liquid to vapour. The pump still works -- it just pumps vapour instead. The pump can't get liquid to pump, every stroke that lowers the inlet pressure just gassifies more liquid, until everything cools down.

This is not the same thing as what you are describing. There is no "lock". The fuel pump is remote from the heat, and is pumping liquid. There may -- or may not -- be gasoline gas in the high pressure portion of the fuel system -- injectors, rails -- but this quickly is purged via the regulator and return line. You don't have to let anything cool to get the engine started.

Faulty parts in the injection system let pressure bleed off when the fuel pump isn't running (I think I mentioned this upthread): leaky injectors, leaky FPR, leaking fuel pump check valve: all of these can allow the fuel system pressure to drop when engine/pump is off, and then heat soak can gassify the gasoline.

But that isn't vapour lock.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

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philip
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Re: Rough running when hot

#21

Post by philip » 13 years ago

asavage wrote:Let's get our terms straight: vapour lock is when the fuel pump lowers pressure on the inlet and the hot fuel changes state from liquid to vapour. The pump still works -- it just pumps vapour instead. The pump can't get liquid to pump, every stroke that lowers the inlet pressure just gassifies more liquid, until everything cools down.
Definition: Vapor lock is the vaporization of solid fuel before the point of admission into a carburetor or fuel injector due to heat. A submersed electric pump some 15' away doesn't get hot enough to fit your definition.

Point in fact, delivery line pressure increases during the heat soak that follows a hot engine shut down. Proof in a carb system is the float bowl needle will be pushed off seat followed by flooding. In fuel injection, line pressure rises -but for- the action of the pressure regulator valve with some leakage relief occuring back through the fuel pump check valve which then leaves vapor at the fuel injector. Engine does not restart easily on vapor. Happens often enough out here near and in the lower deserts where summer temperatures send underhood temperatures through the roof.

Vapor Lock
Definition: When gasoline overheats and boils inside the carburetor bowl or fuel pump of a hot engine, it ceases to flow. This can cause stalling or hard starting. This is called vapor lock, and it usually happens during hot weather. If a hot engine won't start, all you can do is let it sit and cool off. You should check the cooling system to see if anything is causing the engine to run unusually hot (a bad thermostat or cooling fan, for example). Switching brands of gasoline may also help.
asavage wrote:This is not the same thing as what you are describing. There is no "lock". The fuel pump is remote from the heat, and is pumping liquid. There may -- or may not -- be gasoline gas in the high pressure portion of the fuel system -- injectors, rails -- but this quickly is purged via the regulator and return line. You don't have to let anything cool to get the engine started.
Update: The recirculating fuel system design has given way in many cars. My 2003 Corolla is such an example. Fuel gets delivered once ... never to return to the fuel tank. This prevents fuel in the tank from being heated which in turn reduces gasoline vapors delivered to the charcoal canister. This is being done because of the oxygenates in formulated gasolines lowers the "flash point (vaporization point)".

"Vapor Lock" is an old automotive term suggesting vapor trapped that results in fuel starvation until purged.
Last edited by philip 13 years ago, edited 1 time in total.
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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asavage
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Re: Rough running when hot

#22

Post by asavage » 13 years ago

I believe this definition to be incorrect.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

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83_maxima
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#23

Post by 83_maxima » 13 years ago

I replaced all of the flexible lines with the correct I.D. fuel injection line. I also replaced a 24 year old piece about 5 inches long or so, directly beneath the fuel filter.

I will wait to see how it performs after a good drive and a short break.

I believe the problem can probably be attributed to pressure bleed off somewhere.

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kassim503
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#24

Post by kassim503 » 13 years ago

83_maxima wrote: I believe the problem can probably be attributed to pressure bleed off somewhere.
The problem is probably pressure bleed off, try checking the injectors for leakage, along with the FP regulator- also if you feel like checking the check valve, replace the two little hoses around it- its probably never been changed. I would expect one of the three things being faulty and not the hoses, because if the hoses leaked you would probably smell gas, and most likely see puddles of it too. Good practice to change it anyways.

Did you change the injector to rail hoses?
'83 maxima sedan, l24e, a/t, black

227K SOLD 6/7/2012

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philip
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Re: Rough running when hot

#25

Post by philip » 13 years ago

asavage wrote:
I believe this definition to be incorrect.
Like any of these? LOL

Vapour Lock Def
The noun vapor lock has one meaning:

Meaning #1: a stoppage in a pipeline caused by gas bubbles (especially a stoppage that develops in hot weather in an internal-combustion engine when fuel in the gas line boils and forms bubbles that block the flow of gasoline to the carburetor).

Causes:
Most modern engines are equipped with fuel injection, and have a high pressure electric fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapour lock, since the entire fuel delivery system is under high pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapour lock is rare in modern fuel systems. For the same reason, some carbureted engines are retrofitted with an electric fuel pump near the fuel tank.


Chevron's definition
VAPOR LOCK - disruption of fuel movement to a gasoline engine carburetor caused by excessive vaporization of gasoline. Vapor lock occurs when the fuel pump, which is designed to pump liquid, loses suction as it tries to pump fuel vapor. The engine will usually stall, but in less severe cases may accelerate sluggishly or knock due to an excessively lean fuel mixture. Automotive engines are more likely to experience vapor lock during and acceleration that follows a short shutdown period. Vapor lock problems are most likely to occur in the late spring on unseasonably warm days, before the more volatile winter grades of gasoline have been replaced by the less volatile spring and summer grades (see volatility). Vapor lock can also occur in other types of pumping systems where volatile liquids are being handled.

Here's a interesting find. Mixing a little diesel into gasoline to lessen the chances of vapor lock. Hmmm
Mix some diesel into gasoline
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"

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asavage
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#26

Post by asavage » 13 years ago

Both those def's are fine with me. Neither mention fuel boiling the carb as having anything to do with vapour lock :x
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

HowlerMonkey
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Location: West Palm Beach Florida

#27

Post by HowlerMonkey » 13 years ago

If you don't have fuel return, the boiling fuel in the injector rail may not get purged and returned to the tank.

Easy to check.

If your fuel system is bleeding down back through the fuel pump, it will take extra fuel pump run time to compress the vapors down to liquid.

Try this............turn on the car to run and listen for the fuel pump to stop, do it again and turn off the key.

Then quickly back to on and start.

It it fires right up when warm by doing this then either of the above is probably going on.

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83_maxima
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#28

Post by 83_maxima » 13 years ago

HowlerMonkey wrote:If you don't have fuel return, the boiling fuel in the injector rail may not get purged and returned to the tank.

Easy to check.

If your fuel system is bleeding down back through the fuel pump, it will take extra fuel pump run time to compress the vapors down to liquid.

Try this............turn on the car to run and listen for the fuel pump to stop, do it again and turn off the key.

Then quickly back to on and start.

It it fires right up when warm by doing this then either of the above is probably going on.
After trying this yeaterday, after 3 or 4 cycles of the pump, the car will start and run roughly for a short period before it smoothes out to regular idle.

So there is *some* improvement when trying this.

So does this mean faulty fuel pump as in Kassim's case?

IIRC, Kassim put a meter on his rail to monitor the pressure. Maybe I should do this.

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kassim503
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#29

Post by kassim503 » 13 years ago

83_maxima wrote: IIRC, Kassim put a meter on his rail to monitor the pressure. Maybe I should do this.
Yep I did, try to rig one up not just for temporary use, try to make it permanent. Ive found the gauge to be pretty useful while diagnosing problems. Also it looks cool :lol:

Mabye your FP is going out, you really dont know until you put a gauge on and see how slowly the pressure raises while the system is priming.
'83 maxima sedan, l24e, a/t, black

227K SOLD 6/7/2012

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83_maxima
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#30

Post by 83_maxima » 13 years ago

Kassim, I sent you a PM.

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