87x4runner

Hopefully the collective minds can help me out with my demon chasing. I have a 1987 4runner with a rebuild long block with about 10,000 miles on it and started having some running issues after it sat parked for a couple months. The truck seems to be misfiring on only cylinder 4 and it is only noticeable when at idle. I verified it was this cylinder by pulling the spark plug wire and it doesn't idle any differently weather it is connected or not. If i race/rev the engine and disconnect cylinder 4 plug it has a noticeable stumble (hence why i determined it only does it noticeably at idle).

Here is some info:

• New plugs and wires
• EGR removed
• Replaced faulty TPS, new one is within spec per 4crawlers instructions
• Checked distributor cap - looks fine
• Compression on cylinder 4 is 170 psi
• swapped fuel injector connect from cylinder 4 to cylinder 3 to see if it is the connection/ECU/harness... connector 4 works fine on cylinder 3
• swapped fuel inject from cylinder 2 with cylinder 4 - no difference
• disconnected cold start injector
• bypassed the VSV for the fuel pressure regulator
• Replaced all of the smaller diameter vacuum hoses
• started the truck cold and put hand on exhaust manifold, cylinder 4 didnt get hot as quick as the others
• Checked valves all good at .008 and .012 (intake/exhaust)

I'm at whits end with this. Any suggestions? Could it possibly be a faulty fuel regulator and there is a pressure drop at the end of the rail causing cylinder 4 not to get enough fuel? It obviously isn't a clogged injector, and the other cylinders seem to be fine so the fuel pump should be good. Possible vacuum leak?

Dirt Driver

Codes? Are you getting spark to #4? How did the plug look?

87x4runner

I'm not throwing any codes.

When I pulled the old plug it looked fine. The same as all the others, gap seemed fine. I checked the gap on the new ones before putting them in.

I also took an old one from one of the firing cylinders and put it in #4 and it still ran the same.

87x4runner

No other thoughts anyone?

scope103

Put your timing light on #4 (instead of #1, as you would when checking the timing). This is an easy way to see if the plug is firing. Visual inspection is the best place to start, but a bad distributor cap (or bad connection to your new wires) is a possibility. And you've done a lot to eliminate other possibilities.

You could let it run at idle for about 30 seconds, shut it off and pull the plug. If the plug is wet you're getting fuel but no spark. If completely dry (better if you clean the plug first) you may not be getting fuel (don't over-rely on this; it's tough to get the plug out before it will dry itself).

87x4runner

So I pinched off the fuel return line to the tank from the fuel pressure regulator on the fuel rail and the idle went up, seemed like maybe cylinder 4 started firing. Is there any way to see if this fuel pressure regulator valve is stuck open? I removed the vacuum line from it with no change.
Any suggestions? I don't want to replace it unless it's bad at $120 a pop.

jeff34484

If your fuel pressure rose when you pinched off the return to the tank, your pressure regulator is fine. if No.4 started firing better i'd replace the injector before it causes a lean burn in that cylinder and does in the exhaust valve or worse.

Philbert

iTrader: (2)

He said he swapped injectors for #2 and #4 with no noticable difference, as well as the injector harness, to isolate the problem - no changes, so I think that rules out injector failure?

I wish I had a suggestion - all the steps you've taken are what I'd have suggested.

Only thing I would suggest is the cap/rotor...you said you checked the cap....how? They are cheap to replace cap/rotor - maybe that's something?

Good luck!

4runnerx3

Spray some carb cleaner around the injector seals on the intake. If one is leaking it's sucking in air. And also spray some on the intake gasket. If it revs up on smooths out for a second, you have a leak.

Terrys87

iTrader: (1)

87 has the wire clip that holds the injector connector to the injector, sometimes it can make a good connection and then get to big of a gap and break connection. Wonder when you were pinching the line if you bumped it to help make a good connection.

Make sure it is well seated and the wire clip is all the way around the connector. The plastic that the wire goes around can snap off and not lock on as well as it should. May possibly need to replace the connector.

Try pushing and pulling gently on the injector connector and see if that effects idle or even doing it while reving it up.

A better option would to replace it with an 88 or newer connections as they have a squeeze tab. 88 is an odd ball year so may need to go to 89 or newer if you need to go that far.

Just seen the last few post were from recent, but orginal post was 8/12, still will post for some others.

Duffdog

did you check the actual lobe of cyl 4? I had the exact same problem you did, turned out that the cam lobe on only cyl 4 was slightly flattened. I replaced the cam with a TRD unit and never had a problem after that. I suppose this could also be caused by a cam follower that is too worn.

Zatara

Did you ever figure out the problem? I'm having the same issue but can't forgot it out...

87x4runner

The rough idle turned out to be an intake leak where the intake gasket meets the head. Spraying carb cleaner around the head found it. Replaced it with reuseable LCE Engineering teflon gaskets and it now runs great.