What could cause a hot start miss/rough idle that clears up?

[jimtash]

Car sits for around 20 minutes to an hour, sometimes it'll start lightly missing on restart. Never does it cold and 3 times now the miss has almost caused it to stall out as if there was a constant dead cylinder and the idle would surge up and down. If I give it gas it usually clears it up but earlier it took going under load to get it to clear. Have no codes.

 

[rubydist]

my red 94 would do that. I figured it was an injector that would leak down, flooding one cylinder on hot restarts. overnight, the gas will leak out of the cylinder and it will start fine.

 

[jimtash]

Any easy suggestions on how to diagnose the injectors and figure which one could be leaking? Plugs aren't telling me anything.

 

[luigisho]

other than the injector and plugs and wires, i would look at the cam sensor, iac, and maf. Surprised there aren't any codes for a misfire.

 

[jimtash]

I think if the coil is firing properly and the injectors are working, no codes will show up. OBDII will though.

Might actually be a leaking injector. Sometimes it's also hard to start when warm. Plugs are also showing up white. Could be from the O2 sensor reading any extra fuel and adjusting the mixture accordingly.

 

[rubydist]

the engine will run open loop for at least a couple minutes after start, hot or cold, so if the injector is leaking down, that should not result in pcm adjusting mixture.

plugs should look a little white if all is working well.

 

[luigisho]

open/closed loop is what I was thinking about with sensor malfunction.

 

[jimtash]

I'll toss a spare cam sensor if need be but first priority is to pop the fuel rail off and energize the system to see if any are leaking down..

 

[K-Dawg]

My 93 did this as well. Don't think it was the cam sensor, but I never figured it out.

 

[zblackbeast]

Have you checked fuel pressure at the rail before starting it when its warm?

Try cycling the pump a few times before you start it.

 

[jimtash]

OK the injectors don't seem to be leaking into the heads from the tips. What I did find though was the pintle caps were all wet on the outside and the o-rings that fit into the head were all wet on top as well. Couldn't tell if it was gas or oil but I'm thinking the latter because some may have have gotten on them the last time the valve covers were off but there is nothing else wet underneath either. Something definitely is getting past the seals though. Oil or gas or even air.

 

[38SHO]

my car did this the entire time I owned it.... was worse the harder I beat it....

I changed lots of parts for other reasons, even with a different engine

the only things on my car that were always the same....

CCRM, vehicle chassis harness, fuel tank/evap system, fuel lines, and the acutal fuel pump module itself- I have changed the actual pump motor before but not the entire part- I really suspect this has something to do about it, unless by some wierd occurance that its an injector problems and both sets of injectors I used had same exact problem...... or I also kind of suspected some kind of wierd tank/evap system problem could have caused this, the car would build excessive pressure inside the tank IMHO when u would take off the gas cap.......

it never bothered me that much that I went mad scientist on it during my time off, sure seems like a lot of people have had this problem over the years with particular cars and I've never heard of a fix...... it surely has to be something wrong, not all SHO's do this


I should have really tried to determine wich cylinder/cylinders is the cause.... be interesting if everyone who has this problem always has it with the same cylinder....

 

[jimtash]

Did you change out the PCM? Mine also has a lot of vacuum in the fuel system. And one other issue that's been common in two different engines is after the cooling fan shuts off, the idle doesn't have a smooth rhythm but sounds like a cammed up lopey V-8. I can't tune it out via the throttle set screw on the TB and different plugs and wires never made a difference. Different fuel pump didn't change it either. Different IAB didn't do anything. It purrs smoothly until the cooling fan is needed and once it shuts off, it idles like crap until driven for a few minutes and the temp comes down to a certain point.

Another thought is the X2J currently in the car would give me a MAF and TPS code when first installed. I have a spare LOS and no codes ever came up with it. The MAF and TPS are the same yet now with the X2J, the codes no longer appear. The cooling fan idle issue is present with both PCMs though.

 

[rubydist]

sounds like you may have a connector / wiring issue somewhere in the harness - maybe at the pcm, maybe elsewhere.

the mafs and tps are on the same power circuit, so a bad wire or connection in that area of the harness could result in codes sometimes and not others. the same kind of issue could affect starting, depending on what the pcm thinks it sees from the mafs or tps at cranking.

 

[jimtash]

Everything from the MAF to the canister purge valve are on the same power line. Never popped a code with the LOS but did with the X2J. Anyway there are some cracked fuel injector connectors that need replacement while I'm in there. Literally almost every connector, sensor, computer module has been replaced. Figure it out one day.

Took a 9 volt battery to each fuel injector plus Ohm tested them. Spayed carb cleaner into each one and gave them juice and all sucked it in and passed it out the other end normally. Ohms come in at 15.7 for each one. These are the purple tops. The pink tops sitting around show 14.6 Ohms.

 

[jimtash]

Should the FPRs hold vacuum with a hand pump? I have 3 here and all pump up to 15hg's and then rapidly fall to 0. None can hold vacuum and nothing moves inside when vacuum is applied with a pump.

 

[rubydist]

sounds like the fuel pump check valve may be allowing the pressure to bleed down.

try turning the key to on for 3 seconds, then off and repeating that 3 or 4 times before the next hot restart. see if that cures the "I don't want to start" issue.

 

[jimtash]

I have a couple laying around and took a vacuum pump to them. None hold any vacuum. The one still on the fuel rail does the same thing. I'm trying to figure out if they should hold some vacuum or is it normal to bleed down.

 

[luigisho]

It is normal to bleed down.

http://www.enginebasics.com/Engine Basics Root Folder/Fuel Pressure Regulator.html

Fuel pump check valve doesn't sound like a bad diagnosis either.

 

[jimtash]

Should be able to hold some vacuum though shouldn't they? Nor does anything move inside when vacuum is applied to them.

The fuel pump is new Walbro 526 pump. There was still fuel in the rail when I pulled the injectors out even though the car had been sitting for 24 hours. It wasn't under pressure though but what did come out was more than enough to ensure all the injectors had gas getting to them at start up.

Think the LOS PCM will go back in when I button everything back up just to see if the condition persist. Going to recheck all the wiring around the injectors and power supply to the sensors.