Good ol' Random Multiple Misfire

[323kielr]

Hi all,



First time posting here, I've done a lot of reading.



I have 2 gmt400's both OBD II

96 Suburban K1500

2000 K3500

Both have their issues that I am still trying to sort out. I have owned each for about 4 years and my family of 6 loves both rigs.



The 96 burb is having some serious issues that I can't track down. I replaced the head gaskets (AND CRACKED CYLINDER HEADS) on it 2 years ago, along with most of the other normal stuff i.e. injectors, fuel pump, timing chain and a slightly larger cam from summit.

I know that I have 2 cylinders in bad shape from when I pulled the heads a while back, (some pitting) but I figured I'd press on, and it has run remarkably well considering.



This has worked great for two years, then last week at the start of a road trip, she got the shakes. Random Multiple cylinder misfire code. It idles awful, and is down on power, I will also note that there is a change in the induction noise, it sounds very "throaty". Due to our schedule we drove it 1,200 miles and replaced some components along the way, nothing helped and the symptoms did not change. It drove better at high RPM, but still noticably down on power. Exhaust smells like gas. Is burning a bit of oil, but I am not sure that is new and likely due to the pitted cylinders.



Replaced cam sensor, crank sensor, coil, plugs (old ones looked great), I left the wires, they all looked great, cleaned the cap and rotor (they also looked great). I tested the ignition control module and it passed. I sprayed the intake with brake clean to try and find a leak and could not, I may have to double check, because that is my suspicion. I also have the typical O2 sensor codes all the way around, but I have always had those on both my rigs and it never seemed to matter. (No cats)



I unfortunately have 1,200 miles under my belt with it in this condition so I have a pretty good feel for the symptoms, I am surprised it survived, my family and I are thankful, but it did not feel good to push it. I feel at a loss, and I am tired of buying parts.



Any insight or help is greatly appreciated.



-Zeke

 

[k1500 97]

Did you inspect the heads prior to reinstall. Any cracks. Also did you do a injector test. Sounds like your compression is down. What is your compression sequence now. The smell of gas from exhaust is a dead give away of a rich mixture. Which brings me to injector testing. How dose it start good, rough, barley??

 

[323kielr]

When I was doing the head gaskets, I discovered the heads were cracked, I hear this is common due to a higher sand content in the Mexican made castings. So I replaced them with new cast heads, sorry I forgot to mention that. I replaced the injectors at the same time with the updated unit although I did not test them, they have worked great for the past two years, and I have not checked compression, it seems odd that something changed all at once.

There is no noticable blowby either.

Wouldn't bad compression typically be isolated to one or two cylinders, and not the random Multiple deal?

 

[L31MaxExpress]

I have been through 2 caps and rotors in the past 4 months on the Express van. I just ordered a Quicksilver brass terminal cap and rotor kit. After GM went to aluminum terminals in their garbage caps they were providing to Mercruiser, Mercruiser started having their own caps built to the OE GM Brass terminal cap specs. It worked once before for me, then GM discontinued them. I went to MSD and they worked, but the MSD caps look terrible now, so horrible I returned the last one and used a Standard Products unit. The standard products one lasted like 3 months. Hopefully the Merc cap will be of high enough quality to last a while. I have a both P0300 and P0304 pending. Currently I have random spark at #4 terminal. The cap and rotor should be here Friday.

The Merc cap and rotor kit is
Quicksilver 8M0061335

 

[L31MaxExpress]

FWIW the distributor cap and rotors fail in ways that cannot be seen. It is internal plastic failure which results in crossfiring usually.

This is what the typical look of Live Misfire Data with a bad cap.

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[k1500 97]

There is no noticable blowby either.

Wouldn't bad compression typically be isolated to one or two cylinders, and not the random Multiple deal?

ok so bad compression can be anything. One cylinder or across the board. If the engine is tired it will be low across all.

 

[HotWheelsBurban]

You can also have misfires on the Vortecs because of the way the plug wires are routed. I added extra separators on my '97 C3500 crew cab 5.7 to combat this issue; there's several places coming from the distributor cap at the rear of the engine, particularly at #5 and #7, that the wires are close to each other. 5 and 7 are adjacent in the firing order as well as in the block, so this is a common place to create a misfire.
I agree with @L31MaxExpress about the cap and rotor quality, or lack thereof, being a contributing factor on this problem too. I have the United Motor Products blue vented distributor cap and rotor set with brass contacts, on both my trucks, and in both cases they ran better and smoother with the new parts.
My crew cab had been sitting a lot; original owner had passed away and the family hadn't driven it much as it was on their country property in rural Texas. Other than bad paint from sleeping outside and minor body damage, it's practically a new truck with just over 130K on it. My Burb is a little bit more tired at 215K but it still runs well(some piston slap on initial startup like they seem to do with some age).

 

[Jeepwalker]

On a dark night, open the hood (engine running) & look for sparks. Probably wouldn't hurt to ohm out the wires. I second the dist cap issue. It would be good to know compression across all cylinders. Put a vacuum gauge on it and watch the reading and needle behavior. There are some good Y/T videos on vacuum readings. What's the idle and off-idle reading?

Going around the top-side of the engine with a propane torch (length of hose on the end) and go around all vacuum areas, intake man, base gasket, injectors etc.

Also, try unhooking the MAF sensor and see if it smooths out (in open loop). Have you ohmed out the temp sensor or verified cold (ambient) and warm temps? Fuel pressure? Ensure the battery to engine and batt to body grounds are solid. I had a wonky runner once that was due to, of all things, a lousy ground. Sorry if you already mentioned some of these things. I'm just thinking out loud now.

 

[323kielr]

I plan on getting a cap and rotor today, I guess it cant hurt to buy a few more cheap parts. I might as well replace the wires as well. Thank you all for the comments, I will provide an update soon.

Does anyone have a good recommendation on a more capable scan tool for these rigs? I have a decent one that can read fuel trims and such but I am curious if there is better data I could be getting.

Also, I have completely neglected the O2 sensor codes on my rigs. Is this an issue? I had the rears disabled because I pulled the cats. The fronts are also throwing codes though, both low voltage and no activity (PO: 131,134,151,154). I completely unplugged them at one point for diagnostic purposes and there was no change. I have never had a drivability issue in the past with these codes and I have never cared that much because it seems like such a rabbit hole and I dont have to pass emissions.
I do understand that the front O2 sensors control fuel mapping, so maybe this is negligent. I would love to hear the valuable opinions from this group.

 

[L31MaxExpress]

You can also have misfires on the Vortecs because of the way the plug wires are routed. I added extra separators on my '97 C3500 crew cab 5.7 to combat this issue; there's several places coming from the distributor cap at the rear of the engine, particularly at #5 and #7, that the wires are close to each other. 5 and 7 are adjacent in the firing order as well as in the block, so this is a common place to create a misfire.
I agree with @L31MaxExpress about the cap and rotor quality, or lack thereof, being a contributing factor on this problem too. I have the United Motor Products blue vented distributor cap and rotor set with brass contacts, on both my trucks, and in both cases they ran better and smoother with the new parts.
My crew cab had been sitting a lot; original owner had passed away and the family hadn't driven it much as it was on their country property in rural Texas. Other than bad paint from sleeping outside and minor body damage, it's practically a new truck with just over 130K on it. My Burb is a little bit more tired at 215K but it still runs well(some piston slap on initial startup like they seem to do with some age).

x2 on the wire routing issue. Mine is correct and I have wide seperators as well. No lightning show after dark either. I am working on pulling the valve covers. I am going to make sure that everything rocker arm related is correct while I wait for the distributor cap and rotor replacement. I never have done a running valvetrain adjustment on this engine since putting it back together and the valvetrain is sometimes noisier than I feel is normal. That in itself could be causing misfire codes for me. But I have money on it being the cap. I have just experienced too many failures with them to not know how they act when spark starts jumping around inside of them. It typically starts with some weird random jerking at cruising speeds (it can jerk so hard it feels like someone randomly cut the ignition off for a split second), then it moves on to random unlocking of the torque converter while cruising on slight uphill sections of road, then you start noticing an actual misfire around idle. In Park if I bring the engine up to 2,000-2,500 I can hear the random misfire in the exhaust note. If mine keeps eating caps and rotors, I will pull the DUI module and coil off and replace with good used GM ones I have laying around. I might also close the plug gaps from 0.045" to 0.035" given my cylinder pressure to reduce the voltage requirement and thus heat on the cap terminals.

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