Persistent misfire Topic is solved

shegarty · Post by **shegarty** » 07 May 2019, 14:51

Hi all, I am having some difficulty figuring out the cause of a persistent P0301 misfire. It may have been occurring for some time but I cannot be certain as it was previously the wife's daily and I haven't been driving it in a while. I did a plug change to address a hard start issue back in the winter and noticed a slightly different idle after doing so but didn't think much of it at the time. I also have an evap code so the MIL is usually on meaning that I don't know exactly when this became an issue. It is very noticeable now and only seems to occur at idle. Otherwise it pulls just as hard as it always has with consistent revs under load. When I did the plug change a compression test revealed that cylinder 1 was slightly low on compression compared to the other 4 but was within 5-10%. This code is sometimes accompanied by P0505 (Idle control circuit) and P0172 (bank 1 rich).

My first thought was that the idle air control valve might be dirty so I've cleaned that, I also pulled the throttle body off the car and gave it a thorough cleaning and tried adjusting the idle control screw. I've also swapped the MAF and cap and wires with known good spares. I've tried pulling the wire on cylinder one and am getting spark, also the injector seems to be firing and voltages on it match cylinder 2 when running.

At this point am I looking at a faulty IAC? If so, is there any way to test it before stumping for a replacement? I hate throwing parts at problems. Thanks in advance.

Post by **RickHaleParker** » 07 May 2019, 15:27

P0301 is cylinder number one only. It got to be something that is unique to cylinder number one. Pull the coil packs for cylinder one and two and switch them. Clear codes and see if P301 becomes P0302.

shegarty · Post by **shegarty** » 07 May 2019, 15:37

No coil packs, it's a 98. I've already changed the plugs and wires. I also checked the plug gap and switched the 1 & 2 plugs with no change. With the second IAC code and it only showing up in a rough idle makes me think its related to the IAC, maybe cylinder 1 is the persistent misfire because of the slightly lower compression?

Post by **RickHaleParker** » 07 May 2019, 16:19

I missed the '98.

Could be a intake manifold gasket leak at #1. Fire it up and spray some starting fluid around intake manifold port #1. If there is a intake manifold gasket leak at #1, it will suck in starting fluid and you will hear the engine surge. If it does surge, let it scare you and back off on the starting fluid.

Lower compression at #1 could be the cause also.

Post by **850 LPT** » 07 May 2019, 16:38

You mentioned that you changed the plugs and wires, did you replace the cap and rotor as well?

shegarty · Post by **shegarty** » 07 May 2019, 16:53

850 LPT wrote: ↑07 May 2019, 16:38 You mentioned that you changed the plugs and wires, did you replace the cap and rotor as well?

Yep, the original post should have read cap as well. I did not replace the rotor since it has always been a persistent misfire on #1 and the code only pops on idle.

dj_v70 · Post by **dj_v70** » 07 May 2019, 17:43

Disconnect injector electrical for cyl 1 and see if idle changes. If not, you have confirmed problem with cylinder 1. Try swapping injectors with cyl 2 to see if issue moves to second cyl. Also, as RHP suggested, look for intake leak.

shegarty · Post by **shegarty** » 07 May 2019, 18:06

dj_v70 wrote: ↑07 May 2019, 17:43 Disconnect injector electrical for cyl 1 and see if idle changes. If not, you have confirmed problem with cylinder 1. Try swapping injectors with cyl 2 to see if issue moves to second cyl. Also, as RHP suggested, look for intake leak.

Disconnecting the injector on #1 does change the idle and using a rod on the injector I can hear it clicking which confirms that it is working. However I suppose it could still be clogged or partially blocked. If this was the case though I would expect reduced performance under load as well. I will check for an intake leak and post back with what I find.

shegarty · Post by **shegarty** » 07 May 2019, 18:26

RickHaleParker wrote: ↑07 May 2019, 16:19 I missed the '98.

Could be a intake manifold gasket leak at #1. Fire it up and spray some starting fluid around intake manifold port #1. If there is a intake manifold gasket leak at #1, it will suck in starting fluid and you will hear the engine surge. If it does surge, let it scare you and back off on the starting fluid.

Lower compression at #1 could be the cause also.

Definitely an intake vacuum leak but not at the gasket ... it's the GD PCV hose that I already replaced once. Confirmed with a hose on the end of my ear, some carb cleaner and finally an inspection camera. Now, does anyone know how to replace that without removing the entire intake manifold?

shegarty · Post by **shegarty** » 07 May 2019, 18:39

Looks like I might have spoken too soon. Further digging suggests that it might be the bung on the end of the intake. Covering up the PCV hose and elbow and spraying still produces the same result. So, looks like I am pulling the manifold then. Let's just pray that it's not a crack!