Misfiring 454, 1990 non-HEI

Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#1
Make
Chevy
Model
P-35
Year
1990
Miles
84513
Engine
454 VIN N
My 1990 P30 motor home with a 454 TBI is, unfortunately, not giving me a code and I am getting desperate. Desperate enough to change parts I can't prove to be bad. This is something I rarely do! The Betty Boop is not roadworthy.
The spark, at the coil, is as yellow as a daffodil. It starts and runs, but has a random misfire at idle. Under acceleration, when cold, it runs smoothly. As soon as it warms up, it starts to misfire badly at all rpms. After about 10 minutes it becomes undrivable, backfiring and stalling. I have replaced the coil and the distributor. I ran a feed wire directly from the battery to power the coil. No change. I ran a body ground to the coil backet. I have tested the voltage at the distributor, hot and cold. I have tested the voltage at the ECM, in the EST & ESC wires, KOEO, and running and warmed up. I have only one wire reading off spec, the EST bypass, at 1.4-1.6 volts. Spec is 1.0. I haven't found a description of the symptoms that might cause. I am stumped.
 
Joined
Mar 1, 2002
Messages
3,451
Likes
46
Points
48
Location
Kentucky
#2
Watch your injector spray pattern to see if one of the injectors starts to flake out after warmup. This is usually caused by a bad connection at ECM.

Another thing to check would be ohms test at Coolant Temp Sensor for resistance values...I don't remember what the specs are.

An "inline spark tester" might be helpful.

Be more specific in regards to "replaced the distributor". New, used, cap, rotor, module, adjusted/set timing procedure?
 
Last edited:

EricC

Hero Member
Joined
Nov 14, 2006
Messages
434
Likes
3
Points
18
Location
Tulsa, OK
Website
facebook.com
#3
Have you verified lack of spark at the plugs, and have you checked fuel pressure? What did your old distributor gear look like? Did the new distributor come with a new ICM and pickup coil? What do the plugs look like? Plug wires? How did you set the timing after replacing the distributor?
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#4
Watch your injector spray pattern to see if one of the injectors starts to flake out after warmup. This is usually caused by a bad connection at ECM.

Another thing to check would be ohms test at Coolant Temp Sensor for resistance values...I don't remember what the specs are.

An "inline spark tester" might be helpful.

Be more specific in regards to "replaced the distributor". New, used, cap, rotor, module, adjusted/set timing procedure?


I replaced the entire distributor, module, cap and rotor. The timing is controlled by the ECM. I turned the distributor until the engine ran rough in both directions, and set it halfway in between. I believe that is the correct procedure.
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#6
Have you verified lack of spark at the plugs, and have you checked fuel pressure? What did your old distributor gear look like? Did the new distributor come with a new ICM and pickup coil? What do the plugs look like? Plug wires? How did you set the timing after replacing the distributor?

The spark is yellow *at the coil* the wires and plugs are fairly new and the wires ohm out like new, plugs look good. The old distributor gear was fine. The new one came with a new reluctor, pickup and ignition module. Timing is controlled by the ECM.
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#7
When I had a head job around 20k miles ago, the mechanic could not locate a timing mark, so he timed it "by ear" and it ran fine. I just put the new distributor in where the old one was, as close as I could, and did as I described above. Since replacing the distributor made absolutely no difference in the way it acted, I concluded that the timing is close enough for the ECM to deal with it.
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#9
So, are you saying that the fact that the coil is putting out a yellow spark is immaterial to the misfire?
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#10
Also, how would you recommend going about setting the timing without a visable scale?
 

EricC

Hero Member
Joined
Nov 14, 2006
Messages
434
Likes
3
Points
18
Location
Tulsa, OK
Website
facebook.com
#11
I am not, but you haven't checked spark at the plugs yet so it's hard to say anything for sure. If you don't have a spark tester, get a screwdriver or an old spark plug and check it the old fashioned way. The way where you don't get bit is preferable. There are probably two sets of timing marks on the engine, one by the water pump, which is the one you need to use, and one down lower, by the oil pan. There should be a groove in the balancer pulley, mark it with a white paint pen or crayon. don't forget to disconnect the bypass before setting the timing, and disconnect the battery afterward to clear the code.
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#12
I stand corrected. There is a scale. I will go get a timing light and see if I can find the mark.
 
Joined
Jul 2, 2018
Messages
8
Likes
1
Points
3
#13
This is a motor home. I have to use the scale on the bottom because it's the only place you can see the balancer. I prefer white out to mark the balacer.
I have, in fact, checked the spark at the plugs, which is why I then checked it at the coil, to eliminate the cap, rotor and wires from the equation.
The bypass is one wire in a 4wire plug on the module. Will it run with the module unplugged? (Sarcasm alert!)
 
Joined
Mar 1, 2002
Messages
3,451
Likes
46
Points
48
Location
Kentucky
#15
1530577383023.png As stated in previous instructions, you need to find and disconnect the"Set Timing Connector" as one part of the timing procedure. I will try to find a pic of what it probably looks like...
1530578389186.png 1530578534058.png 1530578564961.png