1993 maxima, had the MAP sensor replaced and

The car wouldnt start ( just cranked a lot) I had the car towed to a local mechanic.He replaced the MAP sensor and now Ive noticed 2 new problems.

  1. engine idle rpms are erratic. When the car is started RPMs are at 1,500 after a few minutes they drop to normal range of 700 to 900 RPMs sometimes they drop too low and the car stalls. sometimes the RPMs surge up and down from 800 to 1,200 rpm`s.
  2. very hard shift from 1st to 2nd gear.All the other shifts are normal.

He said that he adjusted the RPMs because they were too high.I dont know how he did that because I thought that you couldn`t adjust them because they are controlled by the computer.

some other info that might help,it has an exhaust leak (which Im going to fix) upstream of the O2 sensor,where the resonator connects to the headers. I replaced a rear brake line and while bleeding the lines I got brake fluid on the ABS wheel speed sensors. I replaced the coolant bypass hose and got water all over part of the engine compartment.It was immediately after I replaced that hose that the car wouldnt start.

could the exhaust leak be the cause of the erratic idle rpm`s?
could water have gotten in a wire connector or shorted something out when I replaced the bypass hose?The bypass hose is directly above the transmission.

My biggest concern is the hard shifting from 1st to 2nd gear.

A lot of older cars have idle adjustment screws and/or mixture adjustment screws

I wouldn’t be too surprised if your car has one or the other, or both

Has the mechanic cleaned the throttle body and the idle air control valve

Has he checked for vacuum leaks. Flat intake gaskets, for example, would lead to false air and a rough idle while cold. Then when everything warms up, they seal a little better, and the engine is running better

just some thoughts . . .

Modern engines – electronic fuel injected, like yours --have to be air tight all the way from the throttle body to the exhaust pipe. Otherwise they won’t work correctly. So before you fret over all the possibilities, get that exhaust leak fixed first.

If the wandering rpm problem continues, my first guess is that the idle adjustment screw is out of whack. Cars of that era tended to have this adjustment, while newer cars it has been eliminated. My early 90’s Corolla has it for example. Sometimes a diy’er or mechanic will adjust the idle adjustment screw in an attempt to solve some problem or another, when that’s not what’s causing the problem. The result is a difficult to diagnose wandering idle rpm. That’s why that adjustment was later eliminated .

What happens, the computer is confused why the car is idling the way it is , so it tries to change the idle to get it back in spec, like by advancing the timing or adjusting the mixture, which throws something else out of whack. And round and round it goes. The computer assumes the idle adjustment screw is set within narrow limits, and if it is set outside those limits the computer won’t be able to maintain a steady idle.

One other idea, it’s possible what caused the engine not to start was indeed the water being spilled on it, particularly the ignition system high voltage parts. And replacing the MAP happened to coincide with when those parts dried out. But the replacement MAP is no good, which could cause a wandering idle too. Try installing the prior MAP. On my Corolla at least the MAP is very easy to remove and replace, takes 5 minutes. And make sure the vacuum line going to MAP from the intake manifold is secure and in good shape, and at both connections.

My biggest concern is the hard shifting from 1st to 2nd gear.

Get your transmission fluid and filter changed…and pray.

Plug wires and distrutor cap need replaced since getting wet killed it.

After driving it for the past week I noticed that its shifting at higher RPMs than it was.It would shift at about 2,000 rpms, but now it doesnt shift until about 2,600 to 2,800 rpms, I think thats why its having hard shifts.
Is there a governor or something on the transmission that might need to be replaced?

Adjusting the idle speed might have thrown off the signal from the throttle position sensor (might be a cable from the throttle body to the transmission in this vintage) which can affect the shift points and more.

The factory service manual should have a procedure for properly setting up the idle adjustment.

@circuitsmith, I think you might be onto something there. last year when I replaced the TPS it was having hard shifts,just like now, after I adjusted the new TPS using an OHM meter the hard shifting stopped.
I wish he would have left the idle speed alone, it was fine,now I have to go and fix something that he shouldn`t have even been messing with.

I’ll go out on a limb here. Is there any chance they replaced the MAF sensor rather than the MAP sensor? I can’t find a listing anywhere for a MAP sensor for this car. An out of calibration MAF sensor could cause the symptoms your experiencing.

Also, raising the shift points can cause hard shifting. It doesn’t mean there is anything wrong with the transmission. This could also be caused by an MAF problem.

I also couldnt find anything about a MAP sensor for this car (1993 maxima SE V6 3.0L DOHC) but thats what he said, and that`s what the receipt says also.

M.A.P sensor-------$136.25
1.5 hours labor

I also realized that its shifting 600-800 RPM higher than it did and he raised the idle RPMs by 700 over the normal idle RPMs. that 700 RPM raise falls right in the middle of the 600-800 higher RPM shifting. Ive been looking to find out how to lower the RPMs but everything Ive found says that you cant raise or lower the RPMs on this car by an adjusting screw on the IACV because there isnt an adjusting screw. I do see a screw on the front of the IACV that's sunk down into a little tunnel like passage.Thats the only screw I see that might be an adjusting screw.

Look for a screw that changes the closed position of the butterfly valve in the throttle body.
Get a helper to press the gas pedal with the engine off.
You’ll see the pulley and shaft turn on the throttle body.
That’s the area to look for the screw.

This is the list of computer control sensors for controlling the powertrain from what I can tell for this engine
1993 Maxima V 3.0L DOHC

Air Flow Meter/Sensor
Air Temperature Sensor ( Ambient / Intake )
Camshaft Position Sensor
Coolant Temperature Sensor/Switch (For Computer)
Crankshaft Position Sensor
Knock Sensor
Oxygen Sensor
Power Steering Pressure Switch
Throttle Position Sensor
Throttle Position Switch
Transmission Position Sensor/Switch
Vehicle Speed Sensor

You’re certain you have the DOHC version, right? I think this car came in a SOHC version too, with the same 3.0L displacement.

yes, its definitely the DOHC engine, and I looked and he did replace the MAF. I put a new MAF on it about a year ago, the MAF thats on there now is marked on the top with yellow paint that says “remanufactured” that`s definitely not the MAF that I put on,I put a new one on.

I managed to get the idle rpms back down to normal by turning the screw on the IACV,but its still shifting at really high rpms.Its seems to be consistant at about 2,800 - 2,900 rpms before it shifts. The car is intermittently running like crap, it randomly stalls will driving,rpms wont go above 2,500, erratic idle. When it starts running like that its acting exactly like the MAF is disconnected. Im going to check the wires to the MAF. Its really aggravating that I paid him to fix it but I`m doing all the work now.

edit: I backprobbed the wires at the MAF with the connector plugged in and the engine running. I got 13.8 volts on the power wire, 0.01 volts on the white signal wire and 0.01 volts on the black ground wire.
so, Im guessing either the sensor is no good or it has a bad ground. Im strongly leaning toward the sensor being bad since it has 13.8 volts going to it but almost no voltage coming out of it to either the ground or the signal wire.

My Corolla doesn’t use an MAF, only a MAP, so I’m not familiar with the techniques w/how to test them. From what you say, it does seem like yours is simply not working for some reason. It seems like the experts here suggest to first try cleaning them, and if that doesn’t work, don’t try to figure out why, just replace.

Usually sensors like those have a 5 volt precision voltage regulator inside. So I’d guess if it was working correctly the output signal would vary from 0 to 5 volts as the air flow increased from none to max air flow. It probably takes some time after it is first powered up to work correctly, as it has to heat up a wire filament first. That’s how they work if I recall correctly, the amount of current to keep that wire filament at a constant temperature is proportional to the air flow. The more air flow the faster the heat dissipation from that wire and the more current required to keep it at a constant temperature. Or maybe it is a pitch and catch arrangement, where the heat from the filament is swept by the airflow to a sensor, forget. Same idea anyway. In any event it won’t put out an output if there’s little to now air flow of course, so you need to have the engine running and warmed up and then test the output at varying rpms I expect. Best of luck.

I ordered a new MAF it should be here early next week, I should take the car back to the guy that “fixed” it, but I dont trust him anymore after this. Im sure it is the MAF because the car runs just as bad with the MAF connected as it does when it`s disconnected.

I think that’s a good plan. Just curious, didn’t your shop say they replaced the “MAP”? Did you ask them about that?


Wanted to mention also that a faulty power steering pressure switch could mess up the idle.