Warm Start Idle Issues

[drcomanche]

I'll try to be as specific as possible for clarity for this, since there are a lot of specific symptoms. For starters, it's a 93 4.0 Manual. Every sensor(IAC, IAT, MAP, CPS, TPS, O2, Coolant temp, etc.), the exhaust all the way back(header, pipes, and muffler. No cat.), the intake manifold is a modified late model, cowl air kit, spark plugs, cap, rotor, coil, wires, are all less than 1000 miles of usage. The fuel system, cooling system, and air system are in the same boat. I basically rebuilt every system on this truck. For a while of that less than 1000 miles, it ran just fine, no breakaway idle, no roughness, no issues to speak of.

 

It started over the past month with it happening when I drove it out for a little and parked it. I would go to the local grocery store or gas station, park, and leave it off for roughly 5 minutes. When I would restart it, the engine would have a slightly noticeable miss for a few minutes then go away(or would just stop having an effect if I started driving). My first thought was heat soak despite the fact that all my injectors were wrapped and the truck never got above 190(the first line on the gauge). Cooling was effective and the outside temp was in the lower 50s, but I gave it a try anyway. I insulated my fuel rail, fuel lines, even the connection, and put a heat shield between the rail and the manifolds. This didn't solve anything. The problem progressively grew worse to where the engine, when restarted, sounding like half the cylinders were firing. Eventually it goes away, but it takes some time and some revving to clear it up. Every time I drove it, it became worse and worse. Now, even when its not running on half an engine, it still has misses at idle. Occasionally, after restart when it's acting up, it will find an idle a around 700 and run comparatively better, and then suddenly cut to 300 and half the cylinders again. Recently, it's been so bad that even revving it and holding at higher rpms doesn't bring them back, it has a ton of misses. No check engine light has been triggered but the truck sounds like it's a carburated drag car without any of the performance. As of now, it hasn't completely stalled itself, but it sure feels close.

 

Things I've done so far: rechecked spark plugs and regapped them. All had a bit of signs of lean running(white dusted) but nothing significant. Replaced ignition coil. Checked all the engine grounds. Checked fuel pressure at the rail(consistent 30 when idling, 28-30 when revving). Pulled fuel rail off and insulated everything. Put in some injector cleaner. Inspected injector wiring and function(all wires and boots look great, can feel injectors firing when running). In the past(still less than 1000 miles), I checked for exhaust gasses in the coolant and it was negative, and also had a buddy help me check compression(150, 150, 152, 153, 150, 152). Checked oil on last change(2 weeks ago), no contamination. Checked the temp sensor(unplugged fans), temp sensor functions. Looked at throttle body, still completely clean from a year and a half ago when I dismantled and cleaned it. Considering everything but the engine itself is new to the truck, I'm at a loss for what could be causing it, but it is quickly becoming worse. On a side not, when I start it after a day, I get no issues like this except a slightly rougher run than usual. It seemed to coincide with the truck itself being warm, but with the temp outside being less than 60, and the stellar performance of the cooling system, I'm not sure what it means.

 

A week ago I replaced the O2 sensor with an NTK one. Today, I replaced the coil, and rechecked all my grounds on the engine side. I then rechecked all my vacuum and am still no closer to an answer.

[cruiser54]

Fuel pressure is low.

[drcomanche]

8 hours ago, cruiser54 said:

Fuel pressure is low.

Isn't 31 the ideal fuel pressure for idle? This isn't a 96+ that operates higher. I can disconnect the vacuum line to the reg and it pushes up to 39. All of this from what I've read was correct on earlier models. Even before this issue started manifesting, the fuel pressure was still reading at 30-31. 

 

Also, out of curiosity, how does the start temperature impact the fuel pressure?

[cruiser54]

Fuel pressure should be 339 kPa ± 34 kPa (49.2 psi ± 5 psi) at idle.

[drcomanche]

Did it change up for the HO engines or the switch to 93? Even on this forum there are old posts placing it at 31 idle 39 without vacuum reg(For up to 92). Every post I've looked up regarding continuing year Cherokees has said that they only went up when the model years hit 96(granted there aren't many pages), which this engine is not. The only late model part on there is the intake manifold, which still runs off the normal injectors and a 92 harness. Even when it ran well, I don't think I ever hit even close to 49. And again, it only does this after it's run for a little while and then shut off. First start idles just fine. Normal rpm for operation it generally does just fine.

[cruiser54]

You are correct. I just went out to the shop and looked in my 1991 FSM. 

[gogmorgo]

Did you check for trouble codes? Could be present even though there's no CEL. 

 

[drcomanche]

20 hours ago, gogmorgo said:

Did you check for trouble codes? Could be present even though there's no CEL. 

 

 

I'll have a look when I start it back up again. I may have to reset the codes, since it probably has a dozen from me plugging and unplugging sensors trying to diagnose it.

[drcomanche]

Had a look today. Cleared all the codes and then hooked it back up and let it run for a bit. Codes were 12(for battery)and 35(I have a custom fan system). Took it for a 45 minute drive and came back and checked again. Same thing.

[drcomanche]

So I decided, since I was on my last legs figuring this out to take the plunge and just pull the head off and hopefully find a problem. Not sure if the gurus here can enlighten me on what a head gasket failure looks like, so I took some pics that maybe someone could help me identify. 

 

Is this crossover? This is in between the 3rd and 4th, and to me it looks like carbon buildup and some burn. 

 

It also looks like there was a bit in between a few more, but there wasn't any noticeable burn on the gasket. If this was the cause, it was very particular, since no other signs point towards head gasket.

 

Also noticed that some of the coolant ports were blocked by the gasket.

 

Held a lot of coolant in the head when I separated them.

 

And, I know it's a bit off this topic, but what is the best way to clean the oil channels? I figure this is the best time to remove the oil grit and chunk that's in there. Too bad I had just done an oil change.