2004 with a misfire under load

2004 with a misfire under load

Postby Mitch [OP] » May 19th 2013, 6:19pm

We have recently upgraded our Astro from our 1999 AWD to a 2004 AWD. I am super happy with it except I am getting a misfire (check engine light flashes) when the van is under load for a duration. We of course use the van to tow our boat, and it does great but when climbing a long hill the check engine light will flash. As soon as we crest the hill, the light stops flashing, and most times it does not even set the a code to keep the light on. The light goes off and we continue driving as normal. The van is now doing this even when not towing. There is a long hill near our house, and when trying to maintain 50-55 up the hill, the light will start flashing about 3/4 of the way up. The biggest problem with this is when the misfire happens and the timing retards the van looses power on the hill. This is a major problem especially when we are towing the boat.
The couple times that it stored a code it was a random misfire, or a misfire cylinder #2. I have tried all the normal fixes, plugs, wires, cap, rotor (all AC Delco parts), and now even a distributor. Fuel pressure measures right and I replaced the fuel filter. A trusted friend of mine who is a mechanic said that he has seen this problem in other astro vans and the cause was a worn timing chain. The slight wear in the chain is enough, when under load, to cause enough of a timing mismatch to cause a misfire. Has anyone else heard of this?
He said that as a temporary fix he has ground out the hole in the distributor clamp and advanced the timing slightly. This was enough to compensate for the wear and temporarily fixed the misfire issue. He willingly admitted this is a band aid fix. So I tried this and the first couple times over the mentioned hill, there was no misfire. However it is still happening and I want to get it fixed right. I am willing to do a timing chain if that is indeed the problem.
I just want to note that other than these hill climbs, the van is running great. Even when the misfire happens, there is no noticeable issue until the light flashes.

So before I go any further, I thought I would throw this up here. Anyone have any insight on this? I am open to any input.
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Re: 2004 with a misfire under load

Postby Leeann_93 » May 19th 2013, 6:47pm

I'd replace the distributor gear or distributor assembly first. The gear is only held on by a pin, but if your cap screw holes are elongated, stripped or cracked in any way, a whole distributor is the way to go.
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Re: 2004 with a misfire under load

Postby Mitch [OP] » May 19th 2013, 7:11pm

That was my most recent try. I replaced the whole distributor assembly. The shaft seemed sloppy, so I put a new one in. The problem is still there.

I forgot to mention there are 115,000 miles on the van.
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Re: 2004 with a misfire under load

Postby Mitch [OP] » May 20th 2013, 3:05am

Any one think it is worth trying different plugs? I thought I read something, I don't know if it was here, that some one installed AC Delco plugs and had a problem. The changed them out for aftermarket plugs and the problem went away.
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Re: 2004 with a misfire under load

Postby Corsemoto » May 20th 2013, 3:20am

I don't think most people here have had any problems with oe type AC Delco spark plugs in their vans. I run AC Delco 41993 in my '04. Could have been a bad plug, got dropped or something I suppose.
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Re: 2004 with a misfire under load

Postby chevymaherchevymaher is online! » May 20th 2013, 3:37am

I would check the color of your spark before doing anything tho. Blue color or look at the coil or distributor module.

Then I would try more octane and a different heat range plug. I have had motorcycles be real finicky about their heat range after changing jets.
Trial and error on the plug tho since I can't read the silly thing.
Since it is pinging it seems like I would go to a range colder first.
Under load it is doing it so my best guess is colder plug. Hot glowing plug causing pre-ignition.

I have owned vehicles both car and bikes, that preferred a certain brand plug as well. I couldn't tell you why. But problem vanished when it had the plugs it liked.
I even switched back with a new plug on a bike just to see and it missed and would barely run after running it hard for 1/2 hour. Different plug and it was fine again.

Try to get it right factory first. But there food for thought on things I went through on tuning before.
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Re: 2004 with a misfire under load

Postby Leeann_93 » May 20th 2013, 3:55am

Mitch wrote:Any one think it is worth trying different plugs? I thought I read something, I don't know if it was here, that some one installed AC Delco plugs and had a problem. The changed them out for aftermarket plugs and the problem went away.



Other way around, yes. All the time. AC Delco solves the problems caused by aftermarket parts.

Out of curiosity, have the injectors been tested? The 2004 has a central metering body with 6 injectors out on stalks. The injector on #2 could be clogged or just failed. Should be tested and, if tests bad, that injector can be replaced by itself.
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Re: 2004 with a misfire under load

Postby RECox286 » May 20th 2013, 3:57am

Have you checked the spark color at each spark plug wire end ? Usually a miss under load is caused by ignition problems.

Also, it would be a good idea, at this point, to check all the high tension pieces and parts by running the engine at night

while spraying a mist of water on everything using ye olde reliable spray bottle, while looking for electrical leaks.

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Re: 2004 with a misfire under load

Postby Mitch [OP] » May 26th 2013, 7:22pm

Well it looks like the problem was a fuel pump issue. The pump was making enough pressure for most driving, but under a duration of load, the pressure would back off. Put a new pump in and the problem is gone...so far.
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