Getting back to this thing now that winter's over (we hope).

This is a 1991 5.2 (318) with the TBI. I'm still working through why this thing is seriously losing power when under heavy load (i.e. pulling a loaded horse trailer). This past weekend my wife and girls barely made it back from a horse show - crawled home at about 40km/hr.

Last fall I was concentrating on the fuel delivery and replaced both the fuel pump and filter. That didn't solve it, so I've just stuffed in new plugs/rotor/cap (though the old ones didn't look too bad) and want to set the timing. My question is what, if anything, I need to do with the vacuum lines while timing it - I don't have a good understanding of how the advance works in this thing. Do any of them need to be disconnected, plugged, etc.?

Also, the CEL blinky-code readout is complaining about the coolant temp sensor (22) and the vehicle speed/distance sensor (15), so I'm on my way out to look at those. Seems to me the latter of the two may be a candidate for causing advance/power problems while on the highway.

 

On my 94 there are two coolant sensors right next to the thermostat housing. One feeds the gauge and I forget what the other one serves. https://www.rockauto.com/en/catalog/...+/+sensor,4748
The code 15 No vehicle speed sensor signal detected during road load conditions. ** or Low output speed sensor RPM, above 15MPH. ** The ** means it will light the CEL

https://www.rockauto.com/en/catalog/...onnector,15798
IDK where it is but there is a replacement option for the connector

 

Yeah, I think this is a common Chrysler thing - one coolant temp "sending unit" to drive the dashboard gauge and a "sensor" to feed the ECU.

But my main question was about timing and vacuum.

 

Sorry I forgot. On my 94 Magnum motor the tune up does not require timing since the computer sets it as it runs.
AFAIK the only thing turning the dist does is set the fuel injector synchronization. There is a setting on the SnapOn MT2500 for setting fuel sync. factory is zero. Some like it to be at +3 for more power. Here is a manual method http://www.bionicdodge.com/Download/...uel%20Sync.pdf

EDIT- Near as I can tell is that to time the 91 TBI you do it normally and disconnect the coolant temp sensor.

EDIT2- I found this The comments seem to be more informative than the video. he seems to have fixed it by removing the NGK plugs and using new Champion plugs. that is the dealer spec. Last comments adds to disconnect vac advance hose when using the light. I guess that is along with having the coolant sensor unplugged.

 

I may be wrong, but I don't think any of that applies to the '91 (which isn't a Magnum). The important difference is that this is the only model year that used a Throttle Body Injector. From what I can see, the timing just needs to be set with an old-fashioned light.

I just checked the coolant temp sensor, and it's reading about 25k ohms (cold). Can anyone tell me whether this is within range?

 

The computer does the timing. There isn't a way to rotate the distributor to adjust the timing. I have the exact truck that I'm finally working on after winter, too. Not running yet, hopefully this week. I've taken the distributor completely out, it just has a bolt to keep it from jumping out.

 

You're confusing me. This distributor has the used-to-be-usual bolt that when loosened allows it to be rotated. The emissions sticker specified 10 degrees BTDC and I got it dialed in, but didn't have the coolant sensor unplugged and will try it again. What I don't understand is what the computer takes as inputs for figuring out the advance.

 

The computer takes a) engine speed, b) throttle position, c) engine coolant temperature, and d) MAP readings to determine the advance. And possibly O2 readings, don't know for sure.

The ECT unplugged causes it to not advance and also runs the idle up to about 2,500 RPM or so (I'd have to check the book).

dmcclelland is talking about the Magnums from 92 up; yours, is not a Magnum.

RwP

 

Actually, on the 1991, yes, the distributor will adjust the base timing that the computer works off of.

And for the 3.9s through 1991.

1992 and up, you're right, it has almost nothing to do with ignition timing. It DOES make a difference for injector timing, however; hence the "injector sync" setting.

RwP

 

Wow - that was almost completely incoherent, and an excellent example of why I hate youtube instructional videos. I really don't know what to make of it - the title says 1990, but he says he just did a motor swap, and it looks like a TBI in there - but TBIs only existed in 1991 (afaIk).