GM 3400 mystery misfire

[Glenn]

I recently did an extensive repair on my 2002 Chevy Venture with a 3.4L engine.I replaced head gaskets, intake manifold gaskets, timing chain and sprockets, including the timing gasket. All ignition components, coils plug wires, plugs, etc., are AC Delco original and new. I went into the engine due to the common intake manifold gasket failure and coolant in the oil.

Here are some symptoms of the “soft” misfire that vehicle experiences. I define “soft” as compared to a harder electrical ignition type misfire. I have included some other information that is not symptomatic, but might be pertinent to trouble shooting the issue.

* The vehicle cranks right away and usually runs smooth at first. Sometimes it will miss a bit right after it starts. At other times it runs very well until warmed up.

* After the vehicle warms up, a rhythmic, ‘loping’ idol sometimes occurs. The car never totally dies. The headlights will slightly dim when the loping occurs.

* The vehicle misses intermittently throughout the driving range.

* It seems to run best at sustained speeds/ interstate, etc.

* The vehicle occasionally stutters at take off in traffic.

* no check engine light has come on.

* vehicle has not totally stalled out.

Any ideas/ trouble shooing pointers would be helpful. I have checked all ground wires. They seems to be tight and in good order.

Thanks much

Topic

[John Hugon]

I would regroup. Put a vacuum gauge on it and see what the vacuum reading is and see what the needle does. I think DFRanger talked about that and I didn’t read a reply.

[jason]

have you checked the voltage on the coil out put? I have had afew of this motor that have had a cracked coil and would cause this code to show.

[jason]

my bad did not c the first there pages of this article. will read everything and get back if I can help.

[Glenn]

JTF, thanks for your helpful advise on my post. Reading some of your other posts, it seems you are familiar with GM’s. I did check the motor with a vacuum gauge per DF Ranger’s suggestion. The needle stays steady within the healthy motor range.This is what caused me to lean away from an internal mechanical issue.
In your experience, have you ever known of an ignition module to cause an intermittent misfire? When I’ve had them fail on GM’s in the past, it’s usually a “no start” situation. I can create the misfire by brake torquing the engine and raising rpm’s. I plan to take out all plugs, wires, (not just front cylinders as before) etc., and inspect for carbon tracking.

[Glenn]

Thanks Hamby…I’m going to check them out anyways. I can’t take anything for granted. My post is starting to read like a small novel now 🙂

[John Hugon]

Yes I have had a bad ignition module cause a miss-fire. The Ignition Control Module’s job is to create a Switching Signal for the Ignition Coils that are sitting on top of it. It’s this Switching Signal that makes the Ignition Coils create Spark.

I was going to copy the diagnosis out of a class book for the diagnosis, but I found this in the internet. http://easyautodiagnostics.com/ignition_gm/ign_module_test_1.php

Use a sewing needed to back-probe the connectors ….don’t pierce the wires for testing.

Do like hambys auto said about checking the coil voltage. I’ve had cracked/good coils spike the module before and caused failures because of high resistance in the secondary circuit.

It would be nice if you had a scope for testing. If you can create a miss-fire under brake torque, you should find what’s going on by checking the coil voltage output hambys auto said during an engine load test.

Yes on worked on GM and some Fords for 38 years only under warranty conditions. After the warranty expired the customers usually had their fill of our service and went to real repair shops.

[jason]

I did read all of this on this 3400 and did not c were a compression test was done. have you did this and was the numbers good?

[Glenn]

Thanks Hamby, I haven’t done a compression test. Before reassembling the engine, I had both heads pressure tested and checked the valves. All seemed to be fine per machine shop. When checking with a vacuum gauge, it stayed steady in the healthy engine range. I didn’t really test engine further. I checked with the vacuum gauge several times. Would you still suggest I do the compression test? I’m going back to some basics, check the voltage output on the coils, etc. thanks for your time.

[jason]

well it would be were i would start on anything that has a hard to find skip. it really is a way to rule out engine problems. i no that you have done alot to find this so i would start back at the beginning and start ruling out problems. i also use a vacuum gauge but have had found that a compression test will tell you more about each cylinder.if this is good i would go back to a spark test to make sure each coil post is putting out like it should if this is ok being the motor it is i would look at the mas air flow sensor and c what the reading is i have had them on this motor and the 3.1 cause them to skip. let me know what u find and i will.help you from there if i can hope this helps.sometimes things like this make you want to burn them up.

[jerry ford]

good day its possible the crank damper could be made a little off on the windoes for the sensor. i had a few in the past cheap ones that when you compared them you could see a difference. hard to check now but you may need to also the code is a evap leak code will not cause a miss the crank sensor is on the rear of this motor 2 wire goes to the ign module yellow and black wire i think.hope this helps.

[EricTheCarGuy]

It’s not likely. The best way to monitor crank sensor output is with a lab scope. Studying the waveform pattern can tell you a lot about it’s operation.

[Glenn]

Thanks for the helpful info…I’ll have an appropriate update soon… I need a little help understanding primary versus secondary ignition in layman’s terms. I get a little lost with some of the in depth explanations on the web. Thanks in advance…

[Glenn]

I’ll try not to make this too long…go grab that cup of coffee and come back 🙂 This past week I went back to some basics. I physically removed each spark plug, wire and coil and visually inspected them for cracks carbon tracking, etc., I then tested each cylinder at the end of the plug wire with my high energy adjustable spark plug tester. I had good, healthy, consistent spark with the tester gapped to 1 inch on each cylinder. Next I did a compression test. Each cylinder seemed to hold compression well. Here are the readings:

Cylinder Dry Wet
1 190 220
2 195 200
3 205 220
4 200 210
5 195 220
6 200 205

I tried to test the MAF sensor, only to realize my ohm meter does not have the Hertz function at the last stage. Here is where things get even more frustrating. I decided to take the car to a reputable GM dealer in my home town. I went to the trouble of typing out the symptoms and all that I had done to the vehicle. The “Advisor” in the service area said this would help the technicians greatly. I asked that they trouble shoot and diagnose the issue.

When I returned to retrieve the car my Advisor friend told me the following (I wasn’t able to converse with the mechanic who analyzed the vehicle)…I was told there is a secondary ignition misfire in cylinder #2. The mechanic temporarily switched out the coil that controls that cylinder and even the ignition module with ones they had in the shop. The misfire was still present. He then proceeded to tell me that I needed to put the exact recommended spark plugs in the van. I had AC Delco platinum ones in it. He said it calls for the iridium rapid fire.

Well you know where this is going… Today, I replaced all the plugs. After my trip to the dealership and their advice I’m out 160 dollars with the same result.

So far for the skipping cylinder we’ve ruled out the plug, plug wire, coil, ICM and apparently the crank and cam sensors (these apparently checked out good at dealership).

So aside from a stick of dynamite, where would you experienced guys go from here? Should I do a leak down test. Is this a possible PCM issue? It could be an injector, but apparently this was checked too. Man, I wish I could have gotten to the mechanic for a personal conversation.

If you’ve gotten this far in my post, then God bless you and thank you…

Whipped and bum fuzzled….

[John Hugon]

The only thing I can say is try to find someone with a lab scope that knows how to use it and attach it to the misfiring cylinder. A good technician can identify a lean running cylinder (if you have one) by witching the firing line.

If you can get a lab scope check all sensors with it.

I had a misfire issue one time (on a 3.1) with the compression readings within specs on the lower end with the misfiring cylinder and found a bent rod, which cause the piston not to reach the top of the cylinder by .002”.

I wish I could help more.

[Glenn]

Wow JTF, wonder how in the world that rod became bent? I’ve never heard of that scenario.. I’d enjoy hearing the whole story. The vehicle is back with my professional mechanic buddy this week. I already owe him 3 steak dinners and counting… 🙂

In the event of a valve possibly sticking a bit, do you have confidence in any products to try such as Seafoam, Marvel Mystery Oil, etc, before tearing into an engine? I know these products are not guaranteed, but I’m wondering if that might be a possible cause.

[Author]

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