'94 FZJ80, power loss above idle (2 Viewers)

[jpoole]

Howdy Mud,

I made the acquaintance of an older man with a '94, US spec, FZJ80 recently. His 80 is way down on power and he's fired the parts blunderbuss at it without success. He's not a computer type of guy so I want to try to bring the power of ih8mud to his efforts via this thread. My current information is a little limited as I've not gotten to look at the 80 but he provided some interesting clues:

The most relevant detail, if accurate, is that timing is dropping to around 8 or 10 degrees ATDC when they throttle up from idle. It sounds like timing was set correctly at 3 degrees, including the jumper step. They then pull the jumper but have kept the timing light out to see how timing advances but have found it instead retards what sounds like about 15 degrees, roughly, as soon as the engine accelerates above idle.​

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My first question for the forum is, what mechanisms lead to timing reduction like this? Knock sensors are the only input signal that I'm aware of off that can trigger a cut like that but I assume there may be others. If this is knock sensor related I wonder if there is a knock sensor issue or if an actual knock issue could be present at such low load/rpms with 3 degree base timing? Can anyone point me to a thread on knock sensor testing for a '94?

Additional backstory on the situation:

• He lives on a mountain that is about a 900 foot climb to the top and then a few miles level driving to his house. On a recent drive up mountain all was fine until about a mile after he had reached the top. After that mile of mostly level/easy driving the engine lost a lot of it's power, and still behaves the same way at this point.
• He's changed oil, fuel filter, fuel pump, AFM, etc. etc.
• He noticed that the top of the original AFM was hot to the touch which is why he replaced it with an apparently aftermarket unit.
• He said temps never left the middle/normal range but he didn't have more detailed temp information other than the stock gauge.
• He has kept all of the original parts

What ideas do y'all have on this? Any other causes for an early FZJ80 to fall flat on minimal accel like this?

Any additional questions that would be easy for me to pass to him or look into if/when I drop by?

I'll add that checking for a torn intake tube seems worthwhile and that based on the hill climb story I'm curious about his fan clutch but I'd guess he would have noticed the intake tube already and that overheating is not currently a factor even if running hot initially triggered the issue. These are two things I'll check if I stop by.

Thanks,

 

[effjay80]

I tried to plug in 2 of the aftermarket / chinese VAFs into my 94 to see if would run correctly and it behaved like that and would simply not run right. I was hoping to find a backup for my original VAF which functions perfectly. That part terrifies me.

 

[mudgudgeon]

Perhaps verify the timing is correct.

When I've time mine once the jumper is out timing varies a LOT.

I'd wonder if free revving the engine without load is going to truly represent what happens with timing during normal driving?

I currently have a knock sensor fault with my 80 that I'm yet to solve(shows as open circuit, new knock sensors). It has other faults too, but doesn't seem to be lacking power.

 

[zapatius]

I would be very skeptical of the aftermarket VAF that he installed. He would be better off with a used good OE.

 

[jpoole]

Thanks, all good ideas that make sense.

@zapatius totally agree on the vaf/afm and I immediately asked if he had kept the old one and let him know how potentially important it is. I've not heard of much, if any, luck with aftermarket afms on these. Any guesses on why the VAF would be hot to the touch? That seems strange to me and makes me wonder if there isn't a problem in a connected circuit that essentially created a short where the VAF was the weak link and was thus cooking. At this point I'm wondering if him simply sending his to the rebuilder that I saw discussed on mud last year, for inspection/service, is a good next step.

@mudgudgeon do you know if the knock sensors can be checked with a simple resistance test? Do you think that simply unplugging the knock sensors could be a good way to eliminate them, or knock, as the cause for the potential timing drop and bogging?

 

[60wag]

Cat plugged up? Any vacuum left above idle?

 

[BILT4ME]

Thanks, all good ideas that make sense.

@zapatius totally agree on the vaf/afm and I immediately asked if he had kept the old one and let him know how potentially important it is. I've not heard of much, if any, luck with aftermarket afms on these. Any guesses on why the VAF would be hot to the touch? That seems strange to me and makes me wonder if there isn't a problem in a connected circuit that essentially created a short where the VAF was the weak link and was thus cooking. At this point I'm wondering if him simply sending his to the rebuilder that I saw discussed on mud last year, for inspection/service, is a good next step.

@mudgudgeon do you know if the knock sensors can be checked with a simple resistance test? Do you think that simply unplugging the knock sensors could be a good way to eliminate them, or knock, as the cause for the potential timing drop and bogging?

Another possibility is that the timing chain has slipped due to the sliders having failed and broken, allowing the chain to be very loose. This could make the time be extremely variable on both acceleration as well as deceleration.

Other than that, I would point to the VAF is not giving the proper signals needed.