GNJ

Im gonna pull the distributor caps off, inspect and check. Thanks Sydwayz and Mike T for the idea. One other thing worth noting is that the tachs are jumping around above 3000 rpm.

Wildman_grafix

Is there a concern that the advance is jumping around on your port motor scan?

GNJ

Yes - one motor shows steady at idle, the other is unsteady and ranges all over from 12-27 .

sutphen 30

I believe thats the "Idle stabilization" program trying to keep the engine running smoothly.

GNJ

Checked both distributer caps and rotors. Checked the grounds in the back of the engine. They all looked OK. Man! this has me stumped.
It seems like both motors are in choke mode. Anybody else think of something to check or try?

GNJ

OK after much troubleshooting, I see that the engine is only producing 7-1/2 – 8 inches of vacuum which seems to be effecting the map sensor. The Map volts at idle is high with engine at operating temp of 140*. With the engine idling, I tried covering the IAC port in the throttle body to see if it would indicate a vacuum leak. Covering the hole about stalls the engine.

RPM / Map Voltage
Idle – 3.80 V
1000 – 3.72 V
1500 – 2.72 V
2000 – 2.11 V
2500 – 2.11 V

500EFI details
525 cam – by Crane – .610/.632 lift 236/242 degrees duration @.050 114* LS
Dart 308 Heads
9:1 CR
AZM throttle body
80 hrs on motors

Compression on each of the 8 cylinders ~120 lbs

With the new information, can anybody suggest what to do next?

GNJ

OK thanks to everyone who offered advice - the issue turned out to be the modules inside the distributor which were not holding timing. Replaced and solved the issue.

mike tkach

seems strange that the both went bad at the same time.

GNJ

I suspect the modules were going bad for a while and since the motors still ran and I'm no expert, it was hard to identify. One indicated -2 to +42 at idle on my Rhinda scantool, the other +8 to +22.

BUP

MAP at idle is way off it should be 1 V at idle fully warm up engine and about 4.8 V at WOT

I would rescan that for sure