LH0 refuses to idle properly

[Crapmaro]

I have an LH0 out of a 1991 Camaro swapped into my 1985 Jeep Cherokee currently using the stock ECU and harness that came out of the Camaro.

I've had the engine in the car since August now and since then it's slowly been getting better as I replace each and every part on the thing like the Ship of Theseus. So far everything I've replaced in rough order are:

- Ignition Module (new from AutoZone, also have the original and a spare from my 305, I've used all 3 and they all act the same)
- Distributor cap and rotor
- Spark plugs and wires
- Idle Air Control Valve (used from junkyard, seems to function the same as the original)
- Coolant Temp Sensor (ECU sensor from AutoZone)
- Injectors (Upgrade injectors from SouthBay)
- Fuel lines (replaced rubber and parts of hard lines with -6AN)
- Intake Air Temp Sensor (cheapo from Amazon)
- O2 Sensor (NTK 21002)

The car ran like shit to start because I didn't have the timing set correctly and the injectors leaked. Fixing those 2 things made a huge difference but it still ran rough when cold, replacing the O2 sensor fixed that and now when it idles it idles smooth. The issue now is that when it gets warm it idles REALLY low or just won't start at all unless I stay on the gas while starting it and hold it for a few seconds after it starts.

It also doesn't like to drop the idle on it's own as it warms up. If I start it cold it idles high until I either get it close to stalling under heavy load or restart the engine and then it'll idle normally.

I'm at quite a loss. I suspect the starting issue might be vapor lock from the fuel lines running close to the crossover pipe, but I'm honestly clueless as to why it won't adjust the idle while running. Any ideas?

[86euro]

Do you have a service manual for the Camaro? There's a detailed procedure to properly set the base idle rpm. That will take care of your warm stalling, assuming everything else is fine.

...Or you can just willy nilly turn up the base idle screw to keep the rpm from getting so low. The ecm will eventually adapt to it.

[86euro]

As for the cold start and high idle speed, I've experienced the same thing with my '86 mpfi 2.8 for the last 20 years and 60k miles. If started on a cold day, it will run a bit unstable at first and then when left to warm up, it ends up with a very high idle. If you shut off the car and then restart it, the idle speed will then be correct. I'm curious as to what the issue actually may be.

[Crapmaro]

The stalling issue was definitely because of the minimum idle screw being backed out too far. I won't get into why, but I had it set very low. Seems like my issue was that I thought it was set correctly because it idled fine cold, and that the stalling when warm was the issue, but now it seems it was the opposite: The car would idle fine warm but had a higher-than-normal cold idle (like 2k rpm). I replaced my MAP, IAT, and IAC with the cleanest components I could find at the junkyard (Really don't feel like throwing new part money on a hunch) and one or more of those components fixed both the high cold idle and the lack of idle adjustment while running. I'm gonna bet that it was a combination of the MAP and the IAC.

But of course issues persist as always. The car still surges when warm, not like a vacuum leak but like the IAC is closing all the way, hanging there for a second, and then opening up some. I've kind of been guessing as to what RPM the thing is running at because when my tach is hooked up it reads very high so I'm kinda just going by ear. I'm going to pick up a laser tach so I can dial it in properly and I'll report back when I have it where it should be.

As for your issue, I wouldn't be surprised if it was a combo of bad IAC and MAP like mine.

[SappySE107]

That isn't an idle set screw. Its a throttle stop. The computer itself will control the IAC for idle unless you open it up too much. Are the injectors the same flow rate as stock? I would get a scanner to see what the computer is doing. Not a code reader but something that can datalog or at least let you capture data on the device itself. Letting these non heated O2 sensors warm up at idle will cause the fuel trims to be wrong until you drive around and clear it out/get the O2 sensor hot.

[Crapmaro]

After a lot of reading about unrelated issues on these forums as well as my Camaro service manual I'm pretty sure I've narrowed in on the issue and sort of "fixed" it.

My ECU is from an automatic car and It's now in a manual car with no gear selector switch wired up, so the ECU thinks it's an automatic car that's constantly in park, and I'm pretty sure this is leading to something weird going on with the EGR valve. I'm still not 100 percent sure how the ECU handles the EGR on these cars because I have read a LOT of conflicting information, but it seems that unplugging the valve has solved my issue. I've driven it a lot since unplugging the EGR and it has not failed to start or idled weirdly since.

I'm happy the issue is fixed but I'm also very curious as to why the surging happened?

[SappySE107]

It should be a digital EGR, not Vacuum, so the computer will open and close it. Not sure auto vs manual is doing anything with the EGR. If it works good with it unplugged, awesome Normally the issue with the EGR is it getting stuck and letting exhaust gas back into the intake when the computer isn't asking. That screws up the MAP sensor reading and doesn't calculate the proper fuel to spray with it expecting no EGR %.

[Crapmaro]

After about a week of driving it like this it has occasionally "surged" but not like before. This time it seems like when I press clutch/shift to neutral the ECU sees the reduced load through the MAP and overcompensates when dropping the idle. Sometimes it'll go as low as 450 RPM but usually only dips to about 600 before returning to 750. Sometimes it'll go right back up to correct speed, others it'll "surge" or "hunt" for the right speed. Not a huge driveability issue but a little anxiety inducing lol. Given that this likely tied to the VSS which my Jeep doesn't have, I'm gonna assume this is just something I have to live with for now...

[SappySE107]

If you can add a VSS so it knows its moving, that might fix it.