RPM Dropout

[Chandar8]

Howdy everyone, it is time to consult the Tbird hivemind. Shit bird has had an issue where when I try and go full throttle itll randomly drop out RPM and then come back into it. Occasionally on cold mornings itll do it as well at around 2k but this is less consistent. The car idles normally, generally drives normally and feels fine. This has only started happening after the PI swap.

 

[1997ThunderbirdLXV6]

That looks exactly like my tach when I was going through that whole crank sensor code episode. Do you have any codes?

I have a thread about it. It came back every few months until one day, the car was completely immobile. I ultimately replaced the entire crank sensor wiring between the sensor and the firewall connector. I also got a new PCM around that time, so it's not entirely clear what fixed it. I'm just glad it's fixed!

 

[Chandar8]

That looks exactly like my tach when I was going through that whole crank sensor code episode. Do you have any codes?

I have a thread about it. It came back every few months until one day, the car was completely immobile. I ultimately replaced the entire crank sensor wiring between the sensor and the firewall connector. I also got a new PCM around that time, so it's not entirely clear what fixed it. I'm just glad it's fixed!

No codes at all. I did re use the crank sensor from the previous engine so maybe its just not playing well

 

[1997ThunderbirdLXV6]

In my case, replacing the sensor didn't do anything. The sensors aren't fragile; the wiring apparently is.

 

[Grog6]

That wiring is a pain because it's a tiny signal, and if you break the foil shield in the harness by flexing it, you get no signal. It's a "twisted pair" of wires, in the harness, with said foil covering them. I'd try a new sensor first, making sure there's no debris stuck to it. It is a coil of wire and a magnet, so shit can stick and kill the signal. Old sensors lose magnetic field over time.
If that ain't it, you gotta strip the harness back, and verify it to the eec.

 

[White Lincoln]

Starvation? Not pulling enough full at that particular RPM? Nah... What happens if you bring the engine up to that RPM slower than a quick rev?

I do know the fast blinking turn flasher means you have a bulb out in one or more of your turn signals.

 

[theterminator93]

Does the SES light come on when it drops out? No tach indication seems to point to an issue with CKPS signal.

If you have a live scan tool, you can log RPM - if it drops out at the PCM too then that's almost certainly the issue.

 

[XR7-4.6]

I lean toward crank sensor wiring as well, I kind of suspect it got damaged during the swap and the vibration/engine leaning at high rpm and load possibly is causing an intermittent open. I actually did have a similar issue with rpm dropout in regular driving that indeed was the issue I had. But yes, datalog it. Does it have a custom tune on it now?


The fast blinker could also be LED bulbs with the stock flasher

 

[Chandar8]

I lean toward crank sensor wiring as well, I kind of suspect it got damaged during the swap and the vibration/engine leaning at high rpm and load possibly is causing an intermittent open. I actually did have a similar issue with rpm dropout in regular driving that indeed was the issue I had. But yes, datalog it. Does it have a custom tune on it now?


The fast blinker could also be LED bulbs with the stock flasher

It has a custom tune now, but it did it before the tune as well. On cold mornings it'll drop out going 45-65 at 2k rpm as well as full throttle. Full throttle is just the most consistent way to get it to drop out like this

Starvation? Not pulling enough full at that particular RPM? Nah... What happens if you bring the engine up to that RPM slower than a quick rev?

I do know the fast blinking turn flasher means you have a bulb out in one or more of your turn signals.

If I bring engine up slowly it's happy, just jamming the throttle or cold mornings it'll does this

Does the SES light come on when it drops out? No tach indication seems to point to an issue with CKPS signal.

If you have a live scan tool, you can log RPM - if it drops out at the PCM too then that's almost certainly the issue.

It has no codes but I don't believe there to be a bulb in the SES light at the moment lol when I have had a scanner tool on the car previously though it had nothing

 

[1997ThunderbirdLXV6]

My crank sensor wiring. The somewhat untidy red wire is the ground for the shielding (which I made from aluminum foil tape). I left extra length on it because I meant to connect it more properly...but then lost interest.

The main thing about your symptom, and mine last year, is the tach drop to a complete zero. Evidently the engine isn't suddenly dropping to zero rpm, so what we have a false signal. My code back then was P0320.

 

[White Lincoln]

Evidently the engine isn't suddenly dropping to zero rpm, ....

The video sounded to me like the engine dropped when the tach dropped. Or was Chandar8 shifting? Sounded to me like the engine dropped at the same time.

 

[1997ThunderbirdLXV6]

Sounded to me like the engine dropped at the same time.

Oh yes, sorry, I didn't express myself clearly.

I meant the engine doesn't drop to zero rpm for a second and go right back to 5,000.

Yes, the actual rpm do drop. But not to zero.

My point was: this symptom is very different from, e.g., a faulty coilpack, where you also get rpm drops, but the tach accurately conveys them.

In this scenario, the tach simply becomes erratic. Essentially, the tach disconnects for a second, because the PCM momentarily doesn't know what the actual rpm is; so it simply shows zero.

 

[Chandar8]

The video sounded to me like the engine dropped when the tach dropped. Or was Chandar8 shifting? Sounded to me like the engine dropped at the same time.

Yeah the tach and the engine RPM drops real low, the tach reads zero but obviously the car is still running. Im going to check the crank wiring ehre really soon