RPMs swing wildly on actron ODB1 scanner

[Lewis E]

I have a question I hope one of the experts here can answer. I have a ’91 2WD Comanche with AT and the 4.0. It’s all stock. Tune up items I have used are all OEM or aftermarket equivalents even down to the Champion copper plugs. I have an actron CP9145 connected to my ODB1 connector. Engine is fully warmed up and it’s throwing no codes. Engine seems to be idling normally. (No misses and at around 750 rpms on the tach. The truck runs and drives well. This is the only Jeep with 4.0 HO I have so I have nothing to compare it with….seems to run pretty good.

 

Here’s my readings from the actron with engine idling (fully warm):

ADPT FUEL - 0

AIS MTR POS - 29

BARO (Torr) - 567.1

BATTERY - 13.8

CHARG GOAL - 13.9

COOLANT (F) - 201

COOLANT (V) - 2.39

ENGINE (RPM) Swings wildly from -2000 to +4000 (this is my question)

IAT SENSOR (V) - 3.65

INJ FW (mS) - 3.8

MAP (Torr) - 144.8

MIN THROT (V) - 0.75

O2 SENSOR (V) - 0.18

SPARK ADV (°s) - 14

SPCTL TGT (MPH) - 0

TPS (V) - 0.76

VACUUM (Torr) - 369.8

VEHSPEED (MPH) - 0

IDLE REQ (RPM) 720

IAT (°F) - 138

 

The reading on the engine RPMs swings wildly from -2000 or so to about +4000. The display changes so rapidly it’s hard to say what it is really. I’m assuming this is not normal. The tach does not swing this way….or it can’t react that fast so just sits somewhere in the middle of the swing. The tach works normally when driving around.

Anyone know what things could be causing this? While you’re looking, how do my other readings look?

The truck’s only gone 65000 miles since new and everything looks to be in really good shape.

Thanks much for any input,

[ghetdjc320]

I’d just ignore the scanner if it’s running smooth. If your ais and tps remain steady then the rpm reading is somewhat irrelevant. It’s nice you have readings on the rest. Just use your dash tachometer or a timing light to know your true rpm. 

[Lewis E]

30 minutes ago, ghetdjc320 said:

I’d just ignore the scanner if it’s running smooth. If your ais and tps remain steady then the rpm reading is somewhat irrelevant. It’s nice you have readings on the rest. Just use your dash tachometer or a timing light to know your true rpm. 

Yeah, I've been telling myself to just ignore it but I'm one of those people that has difficulty with that. When I know something isn't doing what it's supposed to it bugs the heck out of me. Those other readings look OK? Is there somewhere I can find out what are normal ranges for all of them? I know what most of them are but don't know what readings I should see on them. Plus, there's a couple that I don't what they are. I just like knowing stuff like that. It's interesting.

I have the factory manuals but they either don't address them or I just haven't found it yet. Thanks.

[Ωhm]

BARO seems low. Not sure about INJ PW, also seems low. Are you up in the mountains?

[ghetdjc320]

Keep in mind that there are very few devices that can accurately pull data from the sbec2 diagnostic port. There was an older model otc scanner that had a pretty decent Chrysler emulator on it. Otherwise, I’d take all the data you pull with a grain of salt. 

[Lewis E]

5 hours ago, Ωhm said:

BARO seems low. Not sure about INJ PW, also seems low. Are you up in the mountains?

I'm at about 800 feet. I don't know what the normal pressure would be here. I'm near Stuttgart, Germany. I assume INJ PW is the pulse width? What would you normally expect?

[Lewis E]

5 hours ago, ghetdjc320 said:

Keep in mind that there are very few devices that can accurately pull data from the sbec2 diagnostic port. There was an older model otc scanner that had a pretty decent Chrysler emulator on it. Otherwise, I’d take all the data you pull with a grain of salt. 

I got the actron CP9145 based on a post by HOrnbrod from way back. I figured it was probably pretty good based on that. He never posted what a normal range of readings would look like. Maybe for the reason you point out.

[Ωhm]

1 hour ago, Lewis E said:

I'm at about 800 feet. I don't know what the normal pressure would be here

 ≈760torr would be about normal for BARO.

 

1 hour ago, Lewis E said:

I assume INJ PW is the pulse width? What would you normally expect?

Yes.  For RENIX 5.3mS-5.7mS for factory equipped and engine at idle, so I'm not sure for HO.

 

 

For your reading enjoyment. Shows some values.

_JEEP 1991 Sequential Fuel Injection Manual_1.pdf