Strange problem. Idle and 1/4 throttle

[TXSC400]

I am a n00b here and not the automotive world. But I have recently picked up a 1992 SC400 and it is running a little rough and not exactly that great of pick up for the 4.0l v8. Not that I have experienced one in while. Other then my friend has a 1999 GS400 VVTi and my ex's father has a SC400. But it just is not what it should be to me.

Here is the problem:

Idles at 500rpm, and not a constant but about every 3-6 seconds it will shake the car. Then on throttle if I have the pedal down about a 1/4 of he way it will hesitate. Also from a line if I put the pedal to Full throttle it will hesitate. If I go 1/4 throttle and let it shake then go to WOT, it will pick up pretty decent. I come to a stop and it goes back to the shaking issue. Once in a while it will drive pretty well (other then hesitating from stop and the 1/4 throttle issue). I checked the Coils to see what they read but my volt meter may be reading wrong because for the primary test I get 1.2 -1.4ohm on one and 1.5ohm on the other. Which is strange for it should be at 10.2 - 13.8 ohm. I have the dial set to 20 and it goes up from 20 to 200, 2k, 20k and 200k. I tried all the settings without getting a reading close to 10.2 - 13.8ohm.

I may just be doing it wrong... no clue. But I need this car running good for it only has 100k miles on it and it is clean. I am thinking about taking it to get a tune up too... any input?

 

[91_4x4runner]

My *guess* is a vacuum leak.

My 3VZ ran/runs like a turd with both vacuum leaks and exhaust leaks. Make sure all the hoses attached to the plenum are tight and don't have cracks.

 

[TXSC400]

Yeah I will have to check that out... It is just strange because it is only at 1/4 throttle... and does not do it at full throttle.. Another strange thing I noticed was the sound at the 1/4 throttle seems like it misses.. It is kind of a missing exhaust tone rather then a continuous sound.

Hey 91_4x4 Where in SATX are you?

 

[Mr.Phreak]

No one has cleaned the engine bay recently??

 

[chilkoot]

same thing my '93 LS400 did until recently, the vibration at idle is your engine mounts. the hesitation is your transmission mount. i would guess you also have minor vibration in your shifter and steering wheel, again the transmission mount.

about $600 from an indie mechanic for all three, they'll eliminate almost all vibration and assuming your trans has been taken care of (edit: fluid wise) will probably reduce your shifting hesitation too.

the vibration at idle is self-explanatory, your engine is shaking. the hesitation at low throttle is your transmission soaking up energy as it is shaking back and forth against the mount as it shifts(there would be no travel on a good mount). at high throttle you don't notice it because, your transmission has enough force on it to keep it pegged against the mount.

 

[TXSC400]

Chilkoot, I may try that but it does seem it is missing not just shaking... even the exhaust tone has a garble like a V8 that is missing.

Mr. Phreak I am sure they did considering the dealer I got it from detailed the hell out of the engine bay. it was way to clean.

 

[chilkoot]

i didn't notice that mention of a miss.

open your hood at night with the car running and see if there's any arcing in the engine bay, it could be a bad ignition wire. are the plugs original?

if one of your coils were going bad you'd probably lose a whole bank from what I've read.

as a possible cheap solution that couldn't hurt anything, how about a seafoam treatment? it could be a dirty injector.

 

[Mr.Phreak]

If it has been cleaned, check that there isn't any water sitting around the plugs. The caps on the leads aren't the most watertight.

 

[duza]

TXSC400,
I have a similar problem, so look forward to you solving this. I have replaced, igniters, coils, leads, plugs, rotor buttons, distributor caps and even changed over the TPS and there has been no change. With Sideshows' help, I even tested the fuel pump.

I originally thought the idling issue was because of the idle being too low (normal idle is around 700RPM), but it is definitely a miss and sometimes results in the motor stalling at traffic lights. All my little vacuum lines seem to be in good nick and I have put some Wynns injector cleaner in the fuel, all to no avail, so I am a little at a loss and it is beginning to Piss me off. The missing at around 1/4 throttle is slowly getting worse and more frequent, so hopefully it will get to the stage where a mechanic can easily diagnose it, or you find your problem and then I can solve mine.

Sorry I can't offer a solution, but it has cost me quite a bit to eliminate the obvious and it may be better for you to take it to someone who will go straight to the problem and fix it without costing you in labour and parts which don't need replacing.

 

[TXSC400]

duza, I did end up solving the misfire... it was an obvious one for me, when Ipulled the covers over the wires, leads and dizzy one of the wires for the spark plugs was off. I still have not resolved the shutter I get. I now noticed it only happens when it is warm. cold the idle is about 800-900 then when it gets warm it drops to 300-400. it is tough. I am going to clean a few things and seafoam it. hopefully an injector. another thing I am going to try is bypassing the fuel pump ecu to see if that kicks her where she needs to be. the nice thing is the power is now there. I am sure once I figure out the shutter she will be 100%

 

[TXSC400]

Ok after seafoam. Fixed. WTF Over. changed spark plugs and oil just after and the car is like brand new.