2.9 L misfire above 2200 RPM - Is my MAP sensor bad?

[SCMoore]

My 1988 2.9L idles fine. When I get it up over 2,200 rpm it will start to misfire.
I have a question about how a MAP sensor is supposed to act. I'm getting 5v on the power wire. 2.3v on the signal wire with the key on, motor off. If I start the motor, the signal wire stays at 2.3v even when I rev the engine. I assume from what I'm finding on YouTube that the signal should go down to 1.5v with the engine running and go up when you rev the engine.

• Are my results telling me my MAP sensor is bad?
• Would these results cause the misfire at higher rpms?

I have new plugs, wires, distributor cap, fuel pressure sensor and ignition control module.

 

[rusty ol ranger]

My 1988 2.9L idles fine. When I get it up over 2,200 rpm it will start to misfire.
I have a question about how a MAP sensor is supposed to act. I'm getting 5v on the power wire. 2.3v on the signal wire with the key on, motor off. If I start the motor, the signal wire stays at 2.3v even when I rev the engine. I assume from what I'm finding on YouTube that the signal should go down to 1.5v with the engine running and go up when you rev the engine.

• Are my results telling me my MAP sensor is bad?
• Would these results cause the misfire at higher rpms?

I have new plugs, wires, distributor cap, fuel pressure sensor and ignition control module.

Is it throwing a Map code?

 

[SCMoore]

Is it throwing a Map code?

It had a code 18. I cleaned up the bare spout ground wire and re-wrapped the wire. No codes after that.
I suppose I should try the code reader again?

 

[rusty ol ranger]

It had a code 18. I cleaned up the bare spout ground wire and re-wrapped the wire. No codes after that.
I suppose I should try the code reader again?

Yeah. Run them again. The couple MAP sensors ive had fail usually make it idle pig rich.

Check the vaccuum line running to the MAP too

 

[SCMoore]

Yeah. Run them again. The couple MAP sensors ive had fail usually make it idle pig rich.

Check the vaccuum line running to the MAP too

Will do. I'll have to borrow a vacuum gage. what reading should I get from that?

 

[rusty ol ranger]

Will do. I'll have to borrow a vacuum gage. what reading should I get from that?

That i dont know

I just did the scientific method of putting my finger on one end of the tube and blowing in the other to see if air escaped

 

[franklin2]

You can't read voltages on the signal wire from the MAP sensor. As you found out, the two outside wires are 5v supply and ground. The middle signal wire does not send a varying voltage, it sends a square wave frequency signal and the frequency varies with the vacuum.

You can use a frequency counter to check it, or you can hook a tachometer up to check it. The guy in the book says at sea level, with the engine at idle, the tach set on the 4 cylinder scale, the tach should read about 4800 rpm. When you accelerate hard, the vacuum should go down, he says the tach reading should go up.

 

[SCMoore]

Vacuum is good. Still getting a code 18 with key off. key on there are no codes.

 

[rusty ol ranger]

Vacuum is good. Still getting a code 18 with key off. key on there are no codes.

I dont think code 18 has anything to do with MAP. Thats related to coil/distributor

 

[franklin2]

Concentrate on the code 18 and the distributor. I think it's trying to tell you what's wrong.

 

[SCMoore]

So I woke up in the middle of the night and realized, that code 18 was probably left over from the broken alternator wire I fixed prior to picking up a code reader. That was also prior to the misfire starting. I cleared the code, (which I didn't realize I had to do) after reading further into the code reader instruction manual.
I warmed up the engine, ran the code reader and no more 18. No codes at all. But I still have the misfire.
I recently had the in-tank fuel pump replaced because the truck left me sit twice when the tank was below a half and it was hot outside. I assumed the pump was overheating and shut down. I would start right up the next day.
It didn't shut off on me after the pump was replaced. shortly after replacing the fuel pump is when got a check engine light and discovered a wire barely hanging on over by the driver side firewall. I fixed that and replaced the alternator.
The misfire started shortly after that. Could my wire repair be the culprit or would my check engine come on like prior to me fixing it?

 

[franklin2]

If you had a fuel problem, it would cause more than a minor misfire, the whole engine would be having a fuel problem and would be jerking violently. A slight misfire can simply be a sparkplug or sparkplug wire problem on one cylinder, not all 6. Since you have no codes now, I would concentrate of the secondary ignition system and check the timing. And keeping checking for codes, one may pop up later.

 

[SCMoore]

Thanks. I had checked the timing the other day and it seemed fine. When you say secondary ignition system what are you referring to? I recently replaced cap, rotor, plugs, wires. Is there a way to test the coil?

 

[franklin2]

Thanks. I had checked the timing the other day and it seemed fine. When you say secondary ignition system what are you referring to? I recently replaced cap, rotor, plugs, wires. Is there a way to test the coil?

That's what I mean, all the high voltage stuff. You could get a spray bottle of water, wait till nighttime, and with the engine idling, spritz water on the wires and the cap, and the ignition coil and wire and see if you see sparks jumping and it starts missing.