I have a 1984 k5 blazer with a 5.7 liter engine with 4bbl carb.
I just found out that the previous owner cut the connection between the distributor and the ECM. Also, the anti-knock sensor is not hooked up to anything.
The engine runs, but we have always had problems with rough running.
Would this be the cause of the rough running.? Should the engine run in the first place?
Also, the truck pings a lot.
The question arose because the distributor also has a film of oil in the cap so when we took out the distributor, lo and behold, the four wire plug that goes to the ECM was not hooked up to anything.
Yes, the engine could still run, and possibly do so with no troubles. The "ECM" in an '84 didn't have much control over the engine and was more of an accessory for pollution controls.
It may be a 5.7 liter now, but odds are 99 to 1 it came with a 5.0 if it has an ESC module/4 wire distributor.
GM issued a TSB is 1982 and again each year until EFI replaced the carburetors. The TSB authorized disabling the ESC system via cutting the green wire and the black wire in the 4 wire harness and jumpering them together on the distributor side of the harness. The reason for the TSB was numerous complaints due to the ESC system being over sensative to random mechanical noise in the engine causing the spark advance to retard to the point that the vehicles basically couldn't pull cotton out of a Kotex. IOWs, you do NOT want the ESC functional.
If the 4 wire is un-plugged, the engine will not run because the basic timer signal from the distributor pole piece is an open circuit. (the ESC module buffers the pole piece signal) Something isn't kosher here... Is there a 6 pin ignition module in the distributor, or was a 4 pin module swapped in?
See above.
Not surprising. The ESC distributors (w/4 wire ESC plug) were calibrated and meant for a 5.0 liter engine. I'd guess that the 4 bbl carb traces back to a 305 also. You probably have the wrong timing advance and a lean mixture due to mis-matched parts.
Many a C-K pick-up truck and G van were hauled in by the tow truck because the ESC module took a crap, results being; no spark.
The pinging is caused by the lack of ECM. On the 81, it's located above the accelerator pedal. Check to see if everything is still good up there. Should be a load of them at just about any junkyard. I think all of the carb engines with spark control use the same unit. Could be wrong there, but they've all looked alike to me. Shouldn't cost you much of anything, since the junkyards never bother taking these out before crushing them. IOW, don't let em rip you for a couple hundred for this thing. Tough part is finding one with the wires uncut.
That's a likely source of rough running. There's a seal in the bottom of the distributor that's worn out. Given what you have to work with, either get a new distributor, or get a good one at the junkyard with the ECM gear. Rebuilding/repairing old ones isn't worth your trouble.
Thanks for the comments. I have some somments for you and some questions.
Right on the money. The previous owner replaced the 5.0 liter with a 5.7 liter but kept the old quadrajet. I replaced it with a holley rebuilt quadrajet of the same type.
Hmm, I will look for the jumper in my distributor. But I do have a question about the other two wires. The brown wire is a ground, so should it be hooked up to ground? Also, what about the white wire. In my manual, it is labeled as "lo". What do I do with it?
Hmm, you are right, something is not kosher here. I saw the distributor after it was pulled from the engine. The distributor did have the four wire connector. It could be that the mechanic had pulled off the jumpers. I'll ask him. However, I am sure that the ESC controller was not hooked up because the cable going to it is missing. All I have is the ECM, and a connector with about three inches of wire on it - the rest of the harness is missing.
Hmm, I'll look at the old distributor. I'm pretty sure it had four spade connectors on it.
Since the distributor shaft was worn out (oil film in the distributor cap), I decided to rebuild the distributor myself. I went to my local chevy dealer and bought a new shaft (includes new weights and springs), a new module for the distributor and a new vacuum advance. The coil and distributor cap are GM parts and have less than 5,000 miles on them, so I'm not replacing them just yet. I told the parts man about the 5.0 to 5.7 liter conversion so I presume that he gave me parts for a 5.7 liter engine. Would this correct the wrong timing advance problem?
With regard to the carb, when I bought the rebuilt carb at my local parts store, I was told that the same QJ was used on the 5.0 and 5.7 liter engines. Was I misled in this area?
You need to determine if he replaced the original Rochester electronic carb with an aftermarket or older standard carb. I've seen alot of folks do this with the CCC systems; they'll yank and pitch the electronic carb, cut the wires to all of the sensors, and put a standard carb on it.
My guess would be that your carb is out-of whack and running lean, causing the rough idle and pinging. Adjust the mixture to richen her up a bit and see if it makes a difference.
Not too sure about that one, but I had an '85 C-10 with a 4.3, and it had the 4 wire plug from the dist, and the feedback carb, but no O2 sensor, no map sensor, and when I got the truck, the center 2 wires from the dist were cut and spliced together. Now I understand why. :-)
*Since I've never worked in California, the question of CCC on a K series Blazer in addition to a C chassis, K chassis or G chassis is not something I'd attest to one way or the other.
Federal C-K and G chassis trucks went straight from mechanical carbs with mechanical distributors (modified by ESC) to Throttle Body Fuel Injection.
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