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Renix idle miss

MACH90XJ

NAXJA Forum User
Location
Oregon
My 90' w/ auto, 4.0, has developed a stumbly miss at idle only. About 1110 rpm's miss is gone. Kinda of stumbly off the line until it hits that rpm range. Miss is a random bump, bump bump, bump. No pattern to the miss, sometimes its 2 bumps or 3 bumps in a row or none at all for 3 seconds. Idle speed seems normal aswell. Has'nt stalled out at idle either. Just got back in town from a 500 mile trip, around 20mpg. Lots of power, 65 up cabbage hill, did'nt miss a beat. Plugs, cap n rotor newer, aswell as o2 sensor. Had some loose manifold bolts searching for problem, not the first time. No change after re torque. I figure if it did not reseal gasket then off idle and gas mileage would suffer aswell ? Tell me if my thinking is wrong. Ran the tests on the 3 EGR componets, all working. Throttle body squeeky clean. Did the back pressure test per FSM, no blockage. Was suspicious of the temps about 5 degrees warmer, have to chalk that up to the warm weather recently, we hit 80* once again.:laugh2: That test goes through some vacuum measurments at idle and 2000rpm. Idle is about 16 and 2000 rpm is 22 of vacuum. Anything suspicous of the 16 at idle ? I can see the miss in my vacuum reading, vacuum would fluctuate 1/2 up or down to the miss of motor. Idle motor cleaned when I did the t-body. TPS per my scanner reading is acceptable range, actually right where I set it last time. Usually tranny will tell me my TPS is acting up.


Thats about it in a nutshell. Searched til my eyes bled, nothing quite the same. Throw me a bone, something I'm missing here.
 
My 89' was doing that little miss down the freeway, once in a while at first then was cutting out like a bad fuel pump.

This was over a few weeks of DD 100mi round trip to work.

On the last day it was chugging so bad I barely made it to the repair shop.

Thought for sure it was the fuel pump, nope it was the CPS.

That was at 240,000mi. on original CPS and the guy even did the FSB ECU direct harness bypass.

So check your CPS, BTW I'm still running the original fuel pump at 396,000mi.
 
The TPS on an auto trans vehicle has 2 sides to it. One side is for engine management and the other side is for giving input to the Trans Control Unit. They can be tested individually. The engine management side of your TPS can have a bad spot in it while the trans side is fine.

PM me and I'll send you an easy to follow TPS adjustment/diagnostic procedure.

While you're poking around doing the TPS investigation, do this: Back probe the B terminal on the flat 3-wire TPS connector and check how many ohms, if any, of resistance you have to the negative battery terminal. Has to be less than 1 ohm.
 
I can see the miss in my vacuum reading, vacuum would fluctuate 1/2 up or down to the miss of motor.

this sounds like a valve not seating-- random misfire at idle that gets less noticeable with engine speed with a corresponding vacuum fluctuation. I'd run a compression check to help locate the offending cylinder and then perform a leakdown test to locate intake or exhaust valve.
 
I would be thinking EGR or control of the EGR.
 
That sucks when you type a reply and it does'nt post just disapeers. This reply is the shorter version of it.

EGR componets tested already.

Valve not seating, I would think this would be all through the rpm range ? above 1100 rpm's no miss at all.

TPS , pm'ed ya cruiser54.

CPS, did ya get any readings from that marginal CPS ? what were they ? Only had 1 go out on me. It would just the no start randomly until it finally would not start. Don't remember any driveability issues.

Thanks for the replies so far !! :cheers:
 
That sucks when you type a reply and it does'nt post just disapeers. This reply is the shorter version of it.

EGR componets tested already.

Valve not seating, I would think this would be all through the rpm range ? above 1100 rpm's no miss at all.

TPS , pm'ed ya cruiser54.

CPS, did ya get any readings from that marginal CPS ? what were they ? Only had 1 go out on me. It would just the no start randomly until it finally would not start. Don't remember any driveability issues.

Thanks for the replies so far !! :cheers:
PM returned. I'll need your email address to send the instructions. They cover testing both sides of the TPS, unlike most that suggest using the trans side connector for all testing. That can screw you up as I've seen many TPSs that were good on the engine management side but bad on the trans side or vice versa. That can send a guy on a wild goose chase.
 
you're likely not going to feel a marginally bad valve over 1100ish rpm. A compression check isn't too much work and would let you rule it out pretty quickly.
 
you're likely not going to feel a marginally bad valve over 1100ish rpm. A compression check isn't too much work and would let you rule it out pretty quickly.

So that cylinder with a leaking valve should be low on compression all the time right. Why would the miss not be a steady miss, like every stroke its fired. Definately a random miss. I have to dig out the compression checker and take a look see.
This evening was a wash for car work, try again tomorrow night.
 
So that cylinder with a leaking valve should be low on compression all the time right. Why would the miss not be a steady miss, like every stroke its fired. Definately a random miss. I have to dig out the compression checker and take a look see.
This evening was a wash for car work, try again tomorrow night.

Since it appears to be low below 1100 RPM, I would think it would show on a compression test if that's the problem. Shorty's idea of eliminating it as a possibility is good.
 
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