P0300 rough idle on 97 Chevy k1500 305

97 chevy k1500 with 305, engine rebuilt by shop 60k ago, last few months only drove truck 4-5 times, last time driving gave p0306, new plugs, wires, cap, rotor, cleared code, now gives p0300, took intake apart removed spider, sprayed/scrubbed each injector and poppet with cleaner, but no way of cleaning the line between the injector and poppets. No change after cleaning. Also ran injector cleaner through tank. All vacuum hoses look in good order, sprayed carb cleaner around intake to check for leaks at intake/throttle body, no leaks. Checked TPS, IAC, MAF, Coil and fuel pressure, all good except fuel pressure was 52-54 while idle, replaced fuel pump, screen filter and main filter, cleared code, rough start and rough idle still gives p0300. Checked coil with ohmmeter, between posts A & C, Chilton says 0.1 ohms, i got 0.5 ohms, went to autozone and had them check a new coil, it have 0.5 ohms…? Can drive at higher speeds and runs smooth, but idle is poor almost dies.Checked fuel pressure after pump replacement, still ~54 psi. Stumped and don’t want to throw more parts at it without a proper diagnosis. Thanks, hope I gave as much detail as needed. Have a OBDII pocket scan tool, but can’t find manual, will try to find manual and see if I can pinpoint the misfiring cylinders.

Often a 300 code can point to a faulty crank sensor but it could be a major vacuum leak causing this also seeing as how it runs smooth at speed.

Another more sinister cause might be a compression problem. The engine was rebuilt but there are a dozen ways to rebuild an engine and only one of them is the correct method. I’m of the opinion a compression test is a proper first step in diagnosing an engine performance problem.
(And yes, low compression on 1 or more cylinders can cause a rough idle that may disappear when the throttle is opened.)

What would I do? I’d connect a vacuum gauge to an intake manifold vacuum source and see what’s going on as that is much easier than running a compression test. A vacuum gauge will reveal in seconds if there’s a problem although if multiple problems exist it may be more difficult to analyze with a vacuum gauge only.

Even something like a weak valve spring, or springs, can cause a rough idle but if this were the case it should have been caught during the rebuild. This is also something that should show up on a vacuum gauge.
Hope someo of that helps anyway.

I too have 97 chevy 305 1500 4x4. Mine started to miss at idle and under light load. Changed wires,cap, rotor and plugs. No change. Turned out to be the cat coming apart and plugging up the exhaust.

thanks for the input guys, as soon as i get time between work and school i will try a compression and/or vacuum test and also check out the catalytic converter. Is there any good way to check the cat to see if its plugging up?

Also, as a follow up, after installing all the new parts i originally listed, the truck has to be cranked for 2 seconds, sometimes more and it will just barely turn over, but run, verrrrry weak idle, but as it sits over 5 - 10 seconds the idle increases to just under 1k Rpm and stays there, but still sounds rough. Forgot to mention that in original post, don’t know if that may help point to something particular… Thanks again guys

A faulty EGR can cause the problem.

You can do a quick, easy check for exhaust obstruction with the vacuum gauge. See “Scenario 14” toward the bottom of this page: http://www.secondchancegarage.com/public/186.cfm

But I tend to doubt the plugged exhaust. That should end up being most noticeable under load rather than at idle.

Temporarily blocking off the EGR valve wouldn’t be a bad thing to look at.

Do you know what the fuel pressure spec is supposed to be? (I don’t have anyplace handy to look it up). You found it at 52-54 but then changed the pump and still had 54. Is it supposed to be higher? I’d not suspect it as a problem since it too would be the kind of thing most likely to show up as a problem under load.

ok, did a compression test, all cylinders are between 155 and 165psi. Vacuum pressure test reveals a mostly steady 15inches…something very close to scenario 10 - 11 on http://www.secondchancegarage.com/public/186.cfm. I already checked for leaks around the intake and throttle body and plug gaps are all good.

One thing I did not check when I tore apart the intake to clean injectors was the fuel pressure regulator and hoses on the CPFI “Spider” . Found the video below about vehicles with the CPFI systems having similar symptoms. They found the problem(s) to be bad fuel pressure regulator and/or leaking fuel lines into the intake. I know these items were not replaced when the engine was rebuilt.

SOOOOO… my next task is to take apart the intake again. I’ll post what I find.

Let me know if this updated info sparks any new ideas! Thanks.

Ok, I took the intake plenum off again. To do this you have to remove the fuel rail. Once the Plenum was removed, I put the fuel rail back on and turned the ignition to the on(not started) position to kick the fuel pump on. At this point fuel was pouring out of the fuel pressure regulator. I replaced the fuel pressure regulator and put everything back together.

Now… when I bought the new fuel pressure regulator I also bought the upper intake gasket/oring kit that includes new fuel line o-rings, distributor gasket, plenum/intake seal, throttle body seal and spider housing seal. Without thinking, I removed the distributor without marking its location to install the new distributor gasket.

I have found the procedure for realigning the distributor while set at TDC on #1 cylinder. But in the procedure (both online and in Chilton book) it says to align the pre-drilled indent on the distributor gear with the white paint mark on the distributor housing assembly. Then it says, the rotor segment should point to the “8” stamped in the housing. This is not the case for me for some reason. I looked at old rotor and it is identical to the new one and both install in one orientation only thanks to some handy alignment holes. So in summary, when I have the indent hole lined up with the white paint mark, the rotor segment does not point at the “8”.

Can anyone clarify the distributor install instructions? I know that once it is installed correctly you need a scan tool to set the cam retard to within +or- 2 of 0. But I can’t even get the damn truck to start. I’ve tried the distributor install 3-4 times and still can’t get the truck to fire. It doesn’t even try to turn over. At my next convenience I will check all the wiring and make sure I didn’t miss a connection or ground wire etc… but at first glance everything looks to be hooked up.