Diagnosing a Re-Starting Problem

[greg mgm]

I'm trying to figure out why my 78 GMC truck 454 is hard to re start after it warms up. It started doing this a year ago (but just occasionally) and now it's happening almost every time I drive it. In the morning it starts and runs fine. After it warms up, if I stop for gas (or whatever) then go to re start it, it cranks and cranks but doesn't fire up. I end up waiting 5 minutes or so, and eventually it does start. I checked the carb, and it squirts fuel, and the distributor has 12v to it. The coil has been replaced, and I just finished a tune up (cap, rotor, wires, plugs, etc) It has never shut off while running. I suspect an electrical component, because when it finally fires up, it just springs to life...not like it has been flooded. At this point I'm at a loss how to diagnose this. Ideas??

 

[TerryD]

Most parts stores have a means to test the ignition module. Take it in to them and have them check it. They may only want to test it one time, make them do it at least 10. When I worked at Advance Auto, I used to run tests until the module got warm to the touch. I found several that were bad that no one else found because I'd test in this manner.

Have you checked to see if its getting spark at the plugs when it's not starting for you? Take your old lady or buddy with you on a short trip and shut it off. When it won't re-start, pull a plug wire off and stick a screwdriver up in the boot. Hold it about 1/16" off the block (or any well grounded bolt) and have them crank the motor. This should tell you if you are getting fire.

 

[greg mgm]

Hey Terry- Ignition module....good idea. I'll have it tested just like you said. Hopefully I can find a helpful "tester" at a local store who can test it 10 times. Checking the spark is the one thing I haven't done....I'll work on that too. Thanks a lot for the info!

 

[TerryD]

Not a problem. Just make sure they test it until it starts to get warm. They'll usually work fine until they get warm, then the problems start.

 

[Can Can]

In general, most ignition modules that are on the way out start showing symptoms similar to yours after getting heat-soaked.

 

[greg mgm]

GOOD! I'm glad to hear that. Thanks.

I just spent time on the phone asking around for bench testing modules, and Autozone is the only place that tests them in my area...and they said they could do diagnostics on or off the truck, with module in or out... Cool. I also contemplated buying this-

http://actron.com/product_detail.php?pid=16163

but it doesn't heat the module up like the more expensive testers which simulates a running engine. For the price it sure tests a lot of electrical components...... Anyway, I'll have the module tested and report back. Thanks guys!

PS- I've just paid for membership, since you guys are so dang helpful. Gotta love this site!

 

[Can Can]

Keep us posted, brother.

 

[dyeager535]

If it's a factory GM module, many (not sure when this started) have a "button" on the top, center of the module. If that's bulged up, it's supposed to be an indicator of failure. Never had a bad module to confirm that though.

 

[greg mgm]

This morning I removed the module, and it looks like brand new. It's a Standard brand module, and I know Standard isn't bad stuff. However, the
"capacitor and terminal block" that it plugs into looks like total crap. The wires are cracked at both ends, and the plastic is starting to come apart, which is really a good sign. I definitely want something to be bad! Going through my old receipts, I saw that the module had been replaced a few years ago, but the terminal block is original, so no big surprise that it looks bad.
So off to the parts store for testing....I'll post back later.

 

[greg mgm]

Just to double check, I went to 2 Autozones for the testing. Both stores had the expensive module testers and tested it many times, heating the module up to the point of being HOT. But, the module tested good at both places.
So, I picked up a new "distributor capacitor and terminal block" which I hope has been causing the problem. Here's the old one-

The wires were cracked at both ends-

I decided to take a look inside to see what was in there. Here I cut the melted tabs to open it up-

Inside the terminal block is nothing more than wires and connectors, but here you can see how bad the wires are-

It might seem obvious that this is the problem, but after a close inspection, I didn't see wires touching each other. I also considered the condenser as the cause, since a bad condenser stopped my vw baja bug from running. Since the camera was out, I decided to cut open the old condenser with a tubing cutter-

After splitting it open, it looks like rolled up aluminum foil in a can-

The inside bottom of the condenser has a curved washer/ disc, and the center of the "foil pac" has a hole which goes top to bottom.

I can't see how a condenser can go bad since it's pretty straight forward, but they do.....It's interesting to see what's inside.

Anyway, I put the new term block on the dist and it started up like I expected. Tomorrow I'll road test it and check if it re-starts after the engine warms up.

 

[greg mgm]

Well, I'm a happy camper. The truck started and re started instantly, even after a long uphill drive. I stopped right in front of our local mechanics to re start the engine (just in case) but didn't need their services. There's a huge difference in how fast it re starts now. Instantly. COOL! Thanks for the help in this thread.

 

[dyeager535]

Nice pics/writeup, appreciate those.

 

[Blue85]

The "condensor" is just a capacitor. It's a local source for the peak currents to the module (also helps supress RFI). That construction is standard aluminum and mylar. They do get old and stop working eventually, but there isn't always visible damage. It's also possible that those old terminals were getting loose and corroded and didn't make good contact.

 

[greg mgm]

So I thought the problem was fixed (uh oh)
Today the truck had to be towed home after running an errand (with the cabover camper and utility trailer of course) But at least I got to do a test when it wouldn't re start. There was 12v to the distributor but no spark. Half hour later, it started up.
A friend mentioned the magnetic pickup assembly as a possible cause of not re starting. And to mention this again, I had the ignition module tested at two Autozone stores, and it tested good. Right now the engine starts and runs fine, but I'm africkin fraid to drive it! If the module tests good, should I still suspect it as the cause? Or can the pickup assembly cause an intermittant problem like this? I really hate throwing $$ at it.....but I REALLY hate being stranded !!

Ideas?

Blue85 Thanks for your post. My manual says the capacitor suppresses radio noise and does nothing for the ignition. In my VW the capacitor went bad (but looked fine) and the engine wouldn't run because of it. Yes, I thought the connections were bad too!

EDIT- I pulled the distributor out and took a close look at it. The magnetic pickup "magnet" was cracked, the part that twists on the center shaft (under the weights and springs) didn't move to allow for timing advance, and the bearings looked scored. Screw it, I'm buying a new distributor. I guess that should fix it !!