Can't find misfire

[ChevyTech]

5 degrees BTDC is to much for these trucks. Spec is zero.
Was the set timing connector unplugged when the timing was checked?
Was the trouble set by checking the timing cleared from the memory?
Have you checked for trouble codes?

WinALDL is very limited but better then nothing. It can look at sensor data. It is not much good for looking for a problem like a worn distributor.

A surge can be caused by low fuel pressure or a vacuum leak as well as other problems.
How was the fuel pressure tested?
Was a gauged installed with a T fitting?
Was a gauge used on the end of a line?
Was the pressure tested with the engine running?

And I don't have an obd1 connector.

What are you trying to tell us?
Are you saying you do not see an ALDL connector on the truck?
Are you saying you don't have a OBD1 cable to connect it to a computer and use WinALDL?
Does this mean you have not checked for trouble codes?

Have you checked to see if it has a good strong spark?
It could have a weak coil.

You mentioned you were thinking distributor module.
Did you have it tested?

[Chevyman91350]

The timing was set with the connector unplugged. I have checked for codes. When I did the fuel pressure test I turned the key on to run and let the pump cycle. After it shut off the pressure went back down to zero. Is this normal? I put the fuel pressure test before the fuel filter using a t fitting. Starting the engine I got 11 psi. When I reved the engine it stayed there. I don't have a obd1 connector that goes into the truck. The coil is new. It's accell. No I haven't checked distrbutor module. I don't know how. I was going to find one at a junk yard and see of it did the same thing. I also have checked for vacuum leaks using carb cleaner. No change in idle speed.

[Chevyman91350]

I'm going to replace the distributor this weekend hopefully. I just wish my truck would run good

[ChevyTech]

The fuel pump going back down to zero after the pump stops is normal. Some TBI units have a pressure bleed hole in them the bleeds the pressure off. A steady 11 PSI running is good enough.

Many of the bigger auto parts stores in my area will test the distributor module for free if you bring it in. The test results from these stores are not always accurate, and some times the will say the module is good when it is not, but for a free test that may find the problem it is worth a try.

Make sure when you install or reinstall the distributor module that you use heat sink compound under it. The compound helps dissipate heat from the module. Without it, the module will fail.

[Chevyman91350]

Ok so that's good news about the fuel pump. Ok maybe I'll take a ride over to auto zone and have them check it. I have 93 octane gas in it because a few people are telling me to do that and I still see no difference.

[ChevyTech]

The 305 TBI engines have a higher compression ratio then the 350 TBI engine. I have found the 305 trucks benefit from running 89 octane rather then the 87 octane regular, but 93 octane does not seem any better. With the testing I have done 89 octane is the best fuel for the 305 TBI trucks.

I would not put 93 octane in the 350.

[Chevyman91350]

Ok today I found a slight intake leak between where the throttle body is and the intake. I'm still not sure if it solved any issues but it seems to idle better. And it still misfires.

[ChevyTech]

It is common for the base gasket under the TBI to leak vacuum, and yes it does affect the idle quality.

You tested the fuel pressure and compression to be good, so you should concentrate on the ignition parts. Don't through cheap parts at it. As stated, I would have the module tested at this point.

You could spray mist the ignition parts with water, and see if you get any big sparks jumping. Watch for cracked spark plugs too.

Double and triple check the firing order.

You may consider having someone put it on a scope.

[Chevyman91350]

Can I spray the ignition module it's self? What are some ways to test it?

[franken]

Open the hood and run the engine in the dark.
I'd be interested to know how low compression can cause low spark voltage. Seems unlikely.

[anthony1536]

i had a problem a while back with my 1989 350 tbi, it would almost die going down the road with no warning. it then got to the point that it would do it at start up but level off. it would go days sometimes weeks with out doing it and then do it everyday. my problem turned out to be the ignition module. these guys on here helped me alot.

[anthony1536]

Oreilly auto does checks on parts like that. but they checked mine and said it was good. it turned out not to be. i replaced mine and the problem went away

[ChevyTech]

There is a piece of equipment that is made for testing the ignition modules. I actually triggers the module and make sure is works while it heats up the module. A common problem is for the module to fail as it gets hot. Nothing you can do at home compares to this test.

If it does not pass their test replace it for sure.
If it does pass the test, it does not mean it is flawless, and may still need to be replaced.

As stated before, make sure you use heat sink compound under the module.

[Chevyman91350]

The thing with my truck is I don't feel the misfire when I'm driving. It's mostly at an idle. But even if I rev it up to 2000 rpms and keep it there it will misfire. I've never felt it while driving. Would that possibly still Be ignition module?

[Chevyman91350]

I can't remember if I've mentioned this before but when the trucks warmed up and I'm at a stop light or waiting for traffic it will keep idling lower and lower until about 400 rpm and will idle very rough miss badly and sometimes stall.

[Chevyman91350]

I dont know why I didn't think of this before I have a youtube video up. Watch it till about halfway you'll hear the miss in the exhaust. It was idling pretty decent that day to and it wasn't even that good. Here's the video.

http://youtu.be/OPL_UXXf0d4

[Chevyman91350]

Ok so today I was waiting in line for a while after my truck cooled down and it was running great. After I gave it a little gas and let it idle back down it was surging and misfiring again. I'm thinking a possible tps sensor for the surging idle but the misfire I'm still not sure. Anyone have ideas?

[Kudzupatch]

Just need a decent multi-meter to check the sensors. Lots of information on how to check those under the FAQ post. It takes less than an hour to check them and then you can move on something else.

[Chevyman91350]

I've tested most of the sensors. Any one have any more ideas?

[speedygonzales]

Originally Posted by Chevyman91350

The thing with my truck is I don't feel the misfire when I'm driving. It's mostly at an idle. But even if I rev it up to 2000 rpms and keep it there it will misfire. I've never felt it while driving. Would that possibly still Be ignition module?

I've experienced what you describe here. Kinda of like a bub bub bub a bub.

You might have air bubbles in the fuel line. A complete and thorough going over of the injectors and the FPR may make it better.

If it's not a problem when driving, it's not a problem worth loosing sleep over. Time to move on to bigger and worse things.

[Chevyman91350]

What is FPR?

[buddy_1]

Fuel Pressure Regulator