TBI question.

[Crookedaxle]

It still won't take off. It will fire a couple of times on either and then I have to crank it for 20 second or so and then it half assed fire on a couple of cylinder for 10 seconds or so but will never take off from the starter. It was all dried out, burnt the plugs with a propane torch and splashed a half of a shot glass of non ethanol gas in the TBI right off the bat. I replaced the ignition module and coil with cheapy **** this past summer. I took one of the old plugs and gapped it .100" or so and it had a spark. I've did that a few times now to a few different plug wires. It isn't bright blue and too impressive though. I'm taking my old coil and the one that's on it now to have tested in the morning. Are they easy enough to test yourself while I'm sitting here all defeated drinking Pabst Blue Ribbon? BTW, my pump pumped for 20 seconds at key on. I pumped about 2.5 gallons out of the tank that way.

 

[Schurkey]

The typical ohmmeter testing of an ignition coil can tell you that it's bad. It cannot tell you that it's good. In other words, they can fail the test, but they can pass an ohmmeter test and still be bad.

I'm surprised at the pump running for twenty seconds at key-on.

 

[someotherguy]

The typical ohmmeter testing of an ignition coil can tell you that it's bad. It cannot tell you that it's good. In other words, they can fail the test, but they can pass an ohmmeter test and still be bad.

I'm surprised at the pump running for twenty seconds at key-on.

Truck must have originally been sold in a very different part of the country.. 20 second prime sounds like it has the hot fuel module.

Richard

 

[Ken K]

I replaced the ignition module and coil with cheapy **** this past summer.

Was heat transfer grease used under the ignition module? Otherwise they are quick to fail due to heat. Cheap part will cause more problems than it's worth.

Ohm the coil if you don't have access to a coil tester.

So, is it now fuel with water in it or weak spark?

 

[Crookedaxle]

OK, an update. I pulled the coil wire and checked it. 860,000 ohms! No that's not a typo. All the sparkplug wires checked around 8,500 ohms so they're good. The Prestolite wires were new when I got the truck. Hey, I found the smoking gun right? Nope, I replaced it and still the same thing. I still get spark with the junk coil wire or another one I put on so I don't quite understand that.

So last summer when the truck died on me about a mile from the house I tested the ignition module as being bad and replaced it. It still wouldn't put out spark so I replaced the coil without checking it and it ran. I chalked it up as a bad coil that fried the ignition module and never gave it another thought. It ran fine without a hiccup and started multiple times in -20F weather for over 6 months now. So then when I found the bad coil wire I thought maybe that fried the coil both times but I kept the old one and compared it to the new one and they check about the same. Under an ohm on the primary side and 8,100 ohms on one and 8,600 ohms on the other coils secondary side. I tried the old coil on the truck and it didn't help. I know an ohm test will only tell you if a coil is bad and won't tell you if it's good but I get a spark with both of them AND the good coil wire and the bad one. I'm stumped. I was thinking maybe the bad coil wire screwed up both of the coils but they check the same and they both give spark. Even though the spark is blue and will jump a spark plug gapped to about .100" I just don't like the look of it. It doesn't put out a blue SNAP it just kind of fizzles there. Hard to explain I guess. Any ideas now guys? I'm grabbing another coil from the junk yard to try and am replacing the cap and rotor even though they were new and look good. That coil wire had to have screwed something up. I just don't know what and can't figure out why it always has spark with everything I try but won't fire.

 

[Crookedaxle]

Was heat transfer grease used under the ignition module? Otherwise they are quick to fail due to heat. Cheap part will cause more problems than it's worth.

Ohm the coil if you don't have access to a coil tester.

So, is it now fuel with water in it or weak spark?

Yes, I used the supplied grease.

Neither fuel or weak spark.

 

[Crookedaxle]

Truck must have originally been sold in a very different part of the country.. 20 second prime sounds like it has the hot fuel module.

Richard

When you say hot fuel module are you referring to a longer cycle time to prevent vapor lock in a hot environment? The truck was out of Colorado.

 

[Ken K]

Yes, I used the supplied grease.

Neither fuel or weak spark.

I would start with the basic engine mechanical, with compression, running compression. With distubtror cap off, rotate crank normal direction until zero or mark, then backwards, note degrees of movement when rotor moves, stop, take reading. 6-8 degrees = ok, 8 - 12 = too much timing chain slop.

I use a GM spark tester (ST125) this requires the coil to produce a spark that has to create 25k to jump gap for go / no-go. Can be done with a philips screwdriver, but don't drink coffee prior to testing.

There has to be a reason why it won't run on good external fuel source with good spark.

Most good shops and every dealership I worked for paid by commission. I try not to loose my ass on anyones vehicle problem. The book time can be cruel but warranty times hurts.

Although not recommended, I have seen more than once, a tech spraying fuel into the engine, while another tech drive the vehicle into the stall before performing diagnostics. I will keep my eye-brows.

 

[Schurkey]

I use a GM spark tester (ST125)

Mine aren't "GM", but they're the same style. Two calibrations, one for HEI and one for "conventional" ignition with ballast resistor in the power feed.

You must be registered for see images

The HEI version has a recessed center electrode, makes for a longer "gap" and requires higher voltage to fire.

You must be registered for see images

I've bought additional spark testers like these on Amazon. OTC sold them, and so do various "generic" sources. There are other styles of spark testers, including some with an adjustable gap--but these are my "go-to" units. About $12 or so.

There has to be a reason why it won't run on good external fuel source with good spark.

First Guess: Fouled spark plugs, although a weak ignition coil would certainly not help.
Other guesses: No compression, plugged exhaust.

 

[Crookedaxle]

I finally found out what was wrong. Like I said earlier, the cap, rotor and plug wires were new when I bought the truck. I pulled the cap and everything looked clean inside. The top of the rotor looked fine too. Well, when I finally pulled the rotor off of the shaft some plastic shavings fell out of the inside of the rotor and landed on the shaft and top of the reluctor wheel. Well I'll be damned, the plastic indexing rib is sheared off in the shaft. The rotor must have been 10-15 degrees off. Great, it was the cheapest piece of the ignition system I could replace all along.

But the mystery continues, it doesn't explain how it happened in the first place and it doesn't explain why my engine is now leaking oil like a sieve from the bottom of the rear bell housing! It runs fine with no noises. Now WTF?