Gmc 7000

More troubles. After getting some help from our buddy, we saw that the plug wires were arcing in the distributor cap. So, ordered a whole new distributor. Installed it and put on all new plug wires. Started and engine ran poorly still. Attempted to time the distributor, but, since this engine does not have a vacuum line to distributor, you have to unplug a wire to stop it from advancing in order to time it ( I may have said that wrong maybe unplugging the wire stops it from retarding but you get my point). We looked all over the internet and what manual we had and found only one reference to the big trucks (gmc 7000) it said to remove the est wire or electronic spark timing wire. Where would that be? The distributor has 2 plugs going into it. One has three wires the other has 4 wires. Or do you disconnect the wire else where? Such as under the dash? Please give us a little guidance, the 366 big block with the tbi is not too common here anyways. It's also confusing because we found out the truck is a 1989 and the engine in it now is not! It is a 1990. This thing has been a nightmare to work on. All sensors on this thing have been changed except for two that no one knows what they are. It runs very rough and rich still, enough that it will foul the plugs in a very short time. Initial compression tests were good. Sorry for piece mealing out the information. It takes a long time to write out a post like this. Hope someone will have a little guidance! Thanks and I'll try to respond tomorrow as we don't have much to do except the normal chores then all day trying to fix this truck.

18436572 firing order
Distributor turns CCW looking down on it.
#1 on the cap is to drivers side rear (toward the brake pedal)
#1 cylinder is driver's side front
#2 is front passenger's side front

Poor performance can come from bad wires, as well as bad order of connection.
If the wires are good, and correct, I would again suggest checking the fuel pressure. excess pressure is really important.
Getting a real mechanic on site is vital. There are thousands of part replacers. Jim

It should have a decal type tag under the hood with the timing spec. and location of the wire, on a pickup it will be either under the heater/a/c box or under the hood on the right side firewall under a plastic shield.

"After getting some help from our buddy, we saw that the plug wires were arcing in the distributor cap. So, ordered a whole new distributor."

You post here wanting help, yet you blindly buy new parts.

As a person that is not exactly a stranger to dealing with a GM truck of that era your post is bizarre.


You are a person that cannot be helped.

I am sorry I wasted my time reading and replying to your earlier post and this post. GOOD LUCK, you will need it!

The EST wire should be tan and black, located on the firewall behind the dist, to the right. But you should be able to ear time it with or without finding the wire.

Have you checked for vacuum leaks? Those were known for sucking in the gasket under the TB. PCV hose can collapse, rot, leak on the underside.

Try shutting the engine off, look down the TB, see if it is dripping gas. If it is, it needs a rebuild kit.

My 1987 Chev truck had a brown wire out of the harness with a plastic disconnect in the area of the brake booster, unplug to set initial timing and plug back in. I'm sure that engine has a knock sensor with a wire running down to it, maybe screwed into one of the block drains, make sure that knock sensor wire is not close to a plug wire or any other electrical where it can pick up a false signal, I have seen that and it will retard the timing all the way and they will barely run and pull themselves. Is the exhaust free and clear? Who fueled it last, someone didn't by accident fill it with diesel?

I think you will find it inside the cab under the dash on the right side, under the glove box. It will be a brown wire with a white stripe that comes out of the wiring harness, has a connector, and then reenters the wiring harness.

Throwing a whole new distributor on it may compound the issue as you don't know if you caused any new ones by not having it geared right, etc. EVERY small town has someone that does a good job on grain trucks - there are millions of them hauling wheat and folks are keeping them going. Cal the Coop and ask them who does them. I always recommend this one, too- call the bus barn in town. I drove school bus for years. Those boys at the bus barn knew Ford and Chevy gassers like the back of their hand. If you are lucky one of them has been there 20 years and will stop by on his way home. He will have it runnig in 15 minutes for a case of beer. STOP throwing parts at it!

I'm sorry but I have to step in here and make a correction. The post I've read by Janicholson have always had very good information, but when he stated that the distributor rotates CCW as viewed from above that is not the case the rotation is clockwise. (sorry Jim) Please don't get discouraged by folks on here that throw the towel in on you in discust when they think your not following their suggestions to a tee. Sure several of the things that you did I also thought were not necessary, but I'm sure you're doing the best you can. I'll say this my hunch is that the problem is most likely something small and most likely buying major components and replacing them is just throwing good money after bad. The only exception may be if you find that the ecm (engine computer) is bad. Now that does not mean you should go order one. I concer with Jim that you should find a way to test the fuel pressure. I hope you find your problem soon. Sorry this ended up kind of long.

Since the engine has been replaced was the original a 366 or a 427? Did this truck ever run correctly for you before having this problem? If it was originally 427 the ECM may be providing too much fuel for your engine displacement.

You have a fairly simple fuel system on that truck. Once the ignition switch is turned on the ECM looks for a tach signal to start the injectors pulsing. Fuel deliver is based on rpm, the throttle position setting, the coolant temperature, and the manifold vacuum signal from the map sensor. The coolant temperature trims the injector pulse width to give more fuel for cold starts. Starting timing is set by the distributor until the engine rpm exceeds 600 rpm at which point the ECM takes over spark timing. Once the ECM takes over timing, it advances the spark until the knock sensor picks up the spark knock. The ECM then uses this information for base timing control.The O2 sensor does not come into play until the engine enters closed loop. It is a trim device only. Being able to see the actual O2 reading and the injector pulse width can go a long way in chasing down too rich a mixture problem. If the O2 is reading full rich and the pulse width trim is maxed to the lean you are getting too much fuel from somewhere.

ECM problems are more likely to be wiring problems rather than a bad ECM. Since most of the ECM works on voltages of 0 - 5 volts a poor connection or a bit of corrosion in a connector can cause a lot of problems.

Looked it up and the image I found was clear in the rotation (Oh well) Only 2 charts show up clearly each is opposite to the other. Putting in GMC 7000 366 firing order (images) finally found it. Jim