Cause for regular/repeating quick RPM dips?? (1 Viewer)

[Kernal]

I've noticed that if I hold the engine at say 1500 rpms while in Park the rpms will repeatedly drop by about 75 for a second then back up to 1500 for about two seconds, then back down. It will continue doing this as long as I hold the pedal in the same place, like the engine or ECU is hunting for the correct mixture or settings?? This isn't new but seems to be getting a bit more noticeable over the past year. It doesn't happen with normal driving or cruising and hasn't affected performance, normal (good) acceleration with no surging or bucking, and no change in gas mileage, no new codes, no known vacuum leaks, no leaks or cracks in the air intake hose, relatively new plugs, wires, cap, rotor, throttle body cleaned,---. Any thoughts what may be the likely cause; throttle position sensor, O2 sensors, MAF sensor ??
Edit: This is with my stock 96 FZJ-80.

 

[Kernal]

Anyone?

 

[Eicca]

First things first. Year, engine, mileage, mods, etc etc. That way we know what you're working with. Put such information in your signature for best results.

 

[Kernal]

Stock 96

 

[96yotalandcruiser]

Did you have the air conditioner on? It acts like on mine too when turn the a/c on.

 

[95LCMachine]

do you/have you run ethanol? sometimes the grade of fuel you use can cause the engine to "lope". Its just a guess but dirty fuel can do this...
I have noticed that my stock 95 has a little "lope" when I use good gas (89+) for a tank or two then back to the 87.

do you notice anything under power? Any smoke or water from the tail pipe at the time of dip?

Just some quick ideas from a rookie!
good luck.

 

[dfwfo]

Did this just start or been going on for a while, is it getting worse over time? ever at just idle?

 

[Eicca]

I think I remember mine loping a tiny bit when I hold it around 1200rpm. I'll have to go play with it and see what I can come up with.

 

[Kernal]

It's been like this actually for a long time, not recent, but seems more apparent. It doesn't affect drive-ability at all, smooth acceleration, good power, no smoke, water, steam, leaks, stumbling, missing, etc. It does it ever so slightly at idle but doesn't seem as regular or obvious than if I hold the rpms at 1500, then the pattern is clearly regular. It's not the same as when the AC comes on, it's not going up and staying up, it's going down 75 rpm's from 1500, then right back up, then right back down in a regular pattern about every two seconds. Not sure of the correct definition but I would say it's hunting, like the ECU is getting a bad or intermittent signal. Or maybe even the fuel pressure is dropping off, but again, it only drops 50-75 rpms and comes right back up in one second?? I usually use WalMart gasoline which is 10% Ethanol, also put a bottle of a concentrated engine cleaner in the tank, no difference during or after that gas was used up. My only guess is it might be related to one of the major sensors/controls, like IAC, TPS, O2 sensors,---?? I'll start testing them to see what I come up with.

 

[mercfan]

Mine does that too (slightly) - I was hoping it would get resolved once I get through baselining it but so far I've replaced all the sparkplugs, wires, distributor cap and rotor and the slight loping is still present (argh!).

We have no option to get ethanol-free gasoline in my state - it's now the law (thanks to our politicos), so I can't test without it, but the Ethanol theory seems likely.

 

[lndcruz]

Mine does it as well when you try to hold the rpm. Done it since I bought it.

 

[Kernal]

I checked again today; appears that the hunting is worse at 1500rpms, tried it at 1000 rpms, much less change in rpms, same for 2000 rpms and rock steady at idle. Maybe there's a bad spot on a sensor at that pedal position or a glitch in what the ECU sees?? IDK.

 

[96yotalandcruiser]

Mine does that too. It drop ~200 rpm when the a/c is on, or ~50-~70 when the A/C is off. I had the a/c belt off when I was in alaska, no need 4 it up there. Ok, time to tell sum of my story. I bought my land cruiser in nov, 2008 in Medford, organ; then drove it to arcadia,california; then to Valdez,Alaska where I had the whole top rebuilt &valve job and replaced every vacuum line & tube at Toyota in Anchorage as P.M.(preventive matnince) (dec, 2009,and with 175,000 mi on it which is what Toyota recommends too); then to Miami,Florida; then to San Antonio, texas(too hot&dry); then to lake texoma, oklahoma; then to homa, la; then back clearwater,Florida which where I currently reside(if nothing else that should tell about Toyotas reliability and still going strong) Ok, back to my point. It's done that since i bought at the dealership (2008) and have had in and out of several toyota dealerships including anchorage, San Antonio(air conditioner belt), and clearwater; and I think with all the work I've had, especially anchorage, they would have let me know if there was anything wrong.

 

[dfwfo]

If it does it at idle and check engine light comes on and off then good chance you have an ECU issue, common for some of the 80's. Mine did the same, took it to my buddies at Lexus and thats what they told me. I took mine out and had it looked at and they couldnt find anything wrong with it cause its an intermittent problem, but put new drivers(?) in it, I also have an injector issue. Found that out after.

Anyways at idle an injector is being held open slightly and causing the loping, runs fine under load, but will not idle, A/C will help smooth the idle.

 

[OGBeno]

but put new drivers(?) in it

I'm sorry, but maybe I'm a bit slow this morning.

What exactly are you talking about?

There is no proven way to modify the internal code of the 80 series ECM.

 

[96yotalandcruiser]

My check engine has never come on, except in ak when I went through water that was above the bottom of the windshield(why I have a snorkel now, which suprisingly enough increased my mpg by 5!). Water in #6 cylinder. It dried out in about 50 mi and I reset the light and has never come on again.

 

[95LCMachine]

I could be totaly imagining this but, here goes.

Driving through OR yesterday (from Sandy to the Bay Area) I started to notice some of the same behavior that Kernal is inquiring about. Although mine happened at lower RPMS there was a "Hunting" going on. I stopped in Grants Pass and got some petrol. I popped the hood to check the tranny fluid (i though it was a good idea considering the steep climbs and descents) and low and behold the inner belt (alt, power steer, water pump) was shredded. Long story short I replaced both belts and repaired the wires (aux radiator fans, yes the PO put electric fans in place of the motor driven fan and shroud, read: "ass hole") that got chewed up when the belt broke. Instant difference, no lope no hunting and it ran like a top for the rest of the journey.

so my question is this; could a loose belt slipping over the alt fluctuate current to the point of causing the engine to lope?

it's out of left field, but after my fix and another 6 hours on the road this is what I came up with.

 

[96yotalandcruiser]

Hmmm... Maybe, but not likely. Im very interested in wat ever the cause.

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