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Yesterday on the way to work my 2000 Chevrolet Silverado 1500 4WD with the 4.8L V8 Vortec engine started flashing a check engine light. I read it and it was a P0300 Random/Multiple Misfire.

I had experienced a slightly rough idle previously (nothing serious) and had been unable to track it down. I replaced the PCV valve as a course of standard maintenance and it seemed to help get the idle back down, but still slightly rough.

I also replaced the spark plugs and plug wires, and replaced the alternator on warranty for an unrelated issue.

After I got the check engine light, I started doing some more diagnosis with a bluetooth-OBDII reader which is when I found that I had misfires only on cylinders 2, 5, 6, and 8. Since it was on separate banks, I could find no commonality of items to replace, so I tried the following things.

  • replaced coil pack in cylinder 2 because it had the highest misfire count, no effect
  • replaced the EGR valve, also with no effect.
  • did the carb test to look for vacuum leaks in a few places on the intake manifold with no results
  • unplugged to MAF to see if the idle improved and it did not (although I didn't try a load test with the unplugged MAF.)

So I guess I'm stuck and about ready to give up on this one and take it to my mechanic, but I was wondering if there was something else I should try or if anyone has any thoughts.

Side note: I only see the misfires at high RPMs (>1500), so could I drive it home if I take it very slowly and rarely open the throttle?

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  • Did intake gasket fix your problem.Im having the same issues with my 05 Chevy 1500 with 4.3. I have done vac check comp.check fuel pressure and so on. Find compression low on one cylinder so pulled heads had them reworked. Put everything back together still rough idle and P0300 and P0102 codes. Plugs plug wires rotor button and distibutor cap have been replaced. I need advice bad before I set fire to it lol.
    – user12660
    Oct 14, 2015 at 11:42
  • My mechanic ended up replacing the upper intake manifold gasket as well as the coil gaskets (it's all part of the same gasket kit). It did solve the problem. I had my mechanic do it because I didn't have the time.
    – StephenH
    Oct 14, 2015 at 13:17

5 Answers 5

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I think at this point you have three areas to look.

First, double check the entire intake tract for vacuum leaks. Anyplace you could have unmetered air (air entering after the MAF). You'll also want to pull the intake tube and ensure there aren't any cracks in it as well. Make sure all of your clamps are in good working order and are sealing things.

Second, and a lot tougher to diagnose, you need to check the ground situation of the engine. Make sure all of your ground straps are in good working order and well connected. There are plenty of ground straps, so search your engine bay over and see what's going on. You'll also need to ohm out the grounds going to the injectors and coil packs. Check to ensure all of the grounds are good.

Third, it seems to me the crankshaft position sensor or the cam position sensor may be having issues. I'm unsure how to test these to see if they are the issue, so don't go jumping on these first. I know a stealership could scan these for you, but don't get caught up in the R-n-R game (Remove and Replace --- it gets expensive fast). The cam & crankshaft sensors can cause the computer to throw misfire codes if they are intermittent, and not throw the specific code showing these sensors are going bad.

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  • Since the OP is facing misfires above a certain RPM on multiple cylinders I'm inclined to think that fuel delivery is insufficient under high loads. Fuel pump on its way out, perhaps?
    – Zaid
    Dec 26, 2014 at 21:59
  • That, too, could easily be checked by using a fuel pressure guage at the Schraeder and reading it under load. Definitely worth checking before replacing a lot of parts.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 27, 2014 at 3:51
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    I guess if the vacuum leak was large enough, it could affect the regulator. I don't see that as happening, though, mainly because the amount of vacuum loss needed to affect the regulator would have to be substantial, which affect how the entire engine would be running. But on the other hand, a vacuum leak could cause lean conditions which would act like it wasn't getting enough fuel, which would be the same effect as having low fuel pressure.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 5, 2015 at 16:10
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    Yeah, I get that the lean condition would act like low pressure, but I actually measured the pressure and it was low.... But I can see that if the pressure was really bad the fuel pressure regulator might not have enough vacuum to maintain pressure.
    – StephenH
    Jan 5, 2015 at 16:52
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    Getting the car back from the mechanic with the replaced intake manifold on Monday, hopefully the problem is solved...
    – StephenH
    Jan 9, 2015 at 21:37
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I found out that the distributors on the vortecs wear out and cause these problems, some say only 100,000 miles. Mine is 243,000, but a new one fixed it. The OEM are plastic and the shaft starts to wobble as they wear and age and produce misfires.. Got a new one made from aluminum with cap and sensor for $34 on eBay, works great. I first had changed everything from sensors to filters and injectors, but it was the distributor. To set the new one on a vortec you have to use a scan tool, you are really setting the cam sensor since the engine is self timing. Instead I bought an app, Car Gauge Pro and a obd2 blue-tooth adapter I got on eBay (BAFX) and set mine for under $35,

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  • A 2000 Silverado doesn't have a distributor as it has an LS based engine (Gen III small block), but rather has coil over plugs ignition system (separate coils for each cylinder). You are thinking of the older Gen I small blocks.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 3, 2018 at 15:01
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Check the spider injection assembly I had multiple cracked fuel lines . They were dumping fuel into the plenum & not into the cylinder .

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    "...spider injection assembly..." - I'm not sure what you are talking about. The LS1 (LR4 variant in this case) is sequential port fuel injection. Separate fuel injectors get fuel from a fuel rail source. There's no spider injection assembly associated with it. Maybe you are thinking of a different make/model?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Oct 29, 2015 at 16:55
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I had the same problem on my 99 found out my wife had been putting E10 in the tank had to replace my fuel filter and intake gasket along with coil gasket. Turns out my 99 don't like that fake fuel E10.

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    E10 shouldn't give any car a problem, even older ones (much older than yours). Yes, you get worse gas mileage on E10 v 100% gasoline, but it shouldn't affect how it runs otherwise. I'd suggest the real issue you had was you got some bad gas which may have had too much water in it from the E10 absorbing it ... there are a plethora of reasons for it, but E10 in and of itself would not have caused you the issue.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 22, 2016 at 20:43
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Well mostly on any style ls motor hardly ever is a coil pack bad p0300 misfire code crank ur truck up and pull that plug wire off while it’s running make sure u use a good pair of insulated player if it’s sparkin when I hold the plug wire close to it then put spark plugs in it and if by any chance plz don’t trust pre gapped plugs double check them if tha problem consists and has a bunch of codes and backfires out the intake put a fuel pump in it I’ve already ran in to this problem a few weeks back

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    Welcome to Motor Vehicle Maintenance & Repair! While I agree with you generally, I don't think I'd be pulling the plug wires. This can cause the coil electronics to fry. A better solution is to pull the wiring harness connector to the coil itself. It gives you the same knowledge (is the coil working?), without the fear of having the coil getting fried or you getting zapped. Also, your post would be much easier to read if you'd please not use text jargon as well as ensuring punctuation is present. We appreciate you being here and hope to see great things!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 3, 2018 at 14:57

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