Missing at Idle?

[MACH90XJ]

I got this miss at idle,not an even miss like one cylinder,kinda of like bump...bump bump..bump..bump bump bump....bump and so on.Very inconsistent the hotter the motor gets.Power seems ok,but I dont know what a new 4.0 should feel like.Fuel pump whines pretty loud,just throwing that out there.Tps is newer aswell is the CPS,o2,TB cleaned and IAC cleaned,plug wires,cap&rotor,plugs-champion,injectors,fuel filter all done in last 6 months.Pulled plugs and they all were burning real clean and even.My gas mileage really sucks too.I can floor it and it accelerates smoothly no hesitation,except at highway speeds it takes along time to pick up speed above 50mph,have to press down to kick it down to get it to go any faster.

I really havent had time to garage it to pin it down yet,so I take it up deer hunting 2 weeks ago at mainly 4000 to 5000 ft. elevation the idle smooths out,not completely though but a very noticable improvement.I come back down to town,300 feet elevation,missing just like before. Ok so I fugured I was loopy and hallucinated the whole thing,but I go elk hunting in same elevation this last weekend same thing a very soft miss. I come home a much harder miss again.

I have no ideas as to why elevation would help a miss at idle not eliminate it though.Lets here your theories on this one .

 

[t bell]

Altitude changes and it improves. Isn't that related to the MAP sensor. I believe the MAP sensor adjust for altitude or something like that. Anybody else?

 

[satan]

Sounds like a lean condition (think vacuum leak; how's your cruise control and/or heater controls?)? -- the higher altitudes would reduce the available air density moving the mixture back to a richer condition - only thing wrong with that is that your mileage is usually pretty reasonable when lean (aside from the loss of power an larger "right foot" requirements -- which you describe, so it'll still be worth a look!)


I'm assuming a '90 here right?
A look at the MAP (it's vacuum source line leaks all the time on those years) may be warranted... you may wanna suspect suspect an EGR condition as well. Cycle the valve manually at idle to insure that your idle get's worse, then return to where it was before (still could be fouled and/or sticking open slightly)

 

[martin]

Yes, sounds like you are running lean. I posted about using the test set to set the TPS on the Renix XJs in the past couple of weeks.

If you take it to the garage ask them to check your TPS calibartion. My cousin's 85 Vette had a poor idle. He installed a new TPS and used the mulitmeter method in the book. I hooked up the test set - not cheap, got it off ebay - and found it was set too low. We reset it, the idle improved greatly and the run had better accelertion.

Also if you are going high alt make sure you have a high alt CPS on the engine.

 

[MACH90XJ]

Yes it is a '90 w/aw4,4.0 and np231,160,000 miles.

MAP SENSOR,Ill put that on the list of testing,although I have not read up on what this sensors function is.Anyone with a brief explanation of its control on the motor and other sensors if any?I do know that if I unplug map it dies instantly.

CRUISE CONTROL, has never worked probably due to the broken wires in the column control switch.Heater blows hot if thats what you mean.Ill have to test the motor vacuum again,if I remember right 15 lbs is good vacuum?Also my 4x4 is vacuum actuated and it functions perfectly.Im positive this is the original EGR valve now that I think of it is probably past its life expectancy.By the way I tried once to take it off and could not get a torx socket down in there,How do you guys reach those bolts?

MARTIN,I read your post on tps settings,have you taken a volt meter reading after you adjusted with the scan tool?I have a base voltage of 4.66 and have adjusted with volt meter to specs.Tranny shifts fine .High alt. cps,I have read about these here but didnt think I was going high enough.Just what benefit is the high alt one over normal?I mostly run down in the lowlands,downtown portland is like 100 ft. above sea level and my home is about 350ft.

One more sensor is acting up but dont know if its related,The coolant sensor for the electric fan is cycling the fan on and off even when at normal temp 195?but stays on when temps hits 210 on the gauge which is normal.My temp usually hits 210 when just idling around mountain roads.Ive started driving around with hood popped open which seems to help alot when going slow.

Thanks for the info so far !!

 

[satan]

MAP stuff (& other)

OK, the MAP senses the pressure (like a barometer) in your intake manifold below the throttle plate - the data is used for fuel calculations - Based on the temperature of the incoming air, the density of the air (from MAP data), and the throttle position, the ECU can work a little math to figure out how much a given volume of air should weigh (for the air/fuel ratio)...

The MAP is a tiny diaphragm with a displacement transducer (a strain gauge of sorts) - the diaphragm is sealed to a cavity the is at "normal " atmospheric pressure, and the other side of the diaphragm is connected to a port (that on your vehicle connects to a thin hard vacuum line to a port on the TB below the throttle plate) -- when you have a vacuum signal in your manifold, the diaphram is drawn toward the vacuum side by the differential pressure (remember there's a standard atmosphere behind the diaphragm), and the displacement/strain on the diaphragm is measured -- basically based on the material properties of the diaphragm, the pressure differential can be calculated ...

Ensure that you have a good vacuum connection to the sensor, and that the hose is not collapsing -- no leaks !

Typical values for a MAP sensor are--

supply voltage - 4.7 to 5.3 Volts
key on engine not running - very near supply voltage (because there'll be very little strain on the diaphragm - a bit lower voltage up here, I kinda exist in a partal vacuum)
engine running at idle - 0.5 to 1.5 Volts

The MAP has qty=3 terminals (A, B, & C) (yeah - the pins are "A", "B", "C" in order toward the vacuum connection - you SHOULD be able to verify the lettering on the connector and/or the body of the MAP itself)

Pin "A" is the ground for this sensor and is typically the terminal farthest away from the vacuum connection

Pin "B" is the output voltage

Pin "C" is the input voltage

To measure the input voltage you can unplug the thing, BUT it's a good-old time-trick to just measure the output on "B"

KEY-ON NOT RUNNING (back-probe the connector while connected)- IF you get anything less than 4.5 Volts (a little less if you're up here in altitude) then pull the plug to measure the real supply value. You'll wanna verify the ground as well

RUNNING AT IDLE (back-probe the connector while connected) - IF you get more than about 1.5 Volts, AND you're positive that your vacuum line is good, verify the ground and/or replace the MAP

Ensure that your ground is ground -- measure a few of em -- the signal grounds at the computer are the most relevant. The ground connectors on the passenger's side of the 4.0 seem to always suck as can the strap from the head to firewall.

 

[satan]

TPS

that was long - Shifting is only 1/2 of the TPS -- the square connector of the TPS provides data to the TCU for tranny shifting - check the other TPS connector's output - that's where the ECU gets data for engine control