Alaskanbadfish

1993 LT1, in a Jeep, wire harness is missing needed components to be able to scan this engine. New MSD opti, new spark plugs, new wires, new msd ignition, new IAC

Problem I need help with:

After initially starting this engine all is fine, goes up to 1500rpms or so the settles down near 800 after a few seconds (great!). But after about 2 minutes of idling at 800, the rpms start going completely crazy....Up to 1500, back to 800, up to 1800, back to 1000, up to 1500 and so on. This doesn't stop until the engine is shut down. When I give it a bit of gas manually, I can notice a definate hesitation. I figured it was a bad IAC so I just replaced that this morning, and same thing. The 93 doesn't have a maf sensor...what else could this be? What gives the IAC it's signal? Seems to me that the IAC is just opening and closing on its own. I'm very frustrated and need some help.

No known vacuum leaks, vacuum gauge stays steady. Just strange it runs OK for a minute or two before going whonky.

As the idle is fluctuating, I can hear air escaping/entering the throttle body area. Almost like a sucking sound like it's starving for air.

Alaskanbadfish

Also I just replaced all new injectors with the new opti tune up.

timopajala

i had a bad tps sensor that sticked and so the pcm thought i pushed the throttle and feed more fuel. I would check that if you got old sensor in there.

Alaskanbadfish

It is rather old. Guess this will be my next step. Thanks.

Alaskanbadfish

Bout to head to the store to get a new one, but isn't it strange the throttle response and idle are perfect during initial morning start up? I'd figure if vacuum leaks or bad sensors, the thing would be crazy all the time..hmm, I'll let u know later today when I slap that new tps in.

Alaskanbadfish

Just put in the new tps, same problem. I read online there is certain ways to install and initially set up the iac and tps, is thus right? I also read they are plug and play...This surging idle is driving me up a wall, please help..

shbox

Guess I found your thread (you have been emailing me). Speed density is very dependent on engine temperature and MAP readings. Since you have no way to scan, that is going to make it quite difficult to take a look at those readings. Your O2 readings need to be observed, as well.

hrcslam

I agree, OP, you really need to get a data log of it. You need to know if it's the IAC or the O2s.

Personally, it sounds like an exhaust leak of some sort. An exhaust leak even after the 02s can cause issues. What is your exhaust set up?

Alaskanbadfish

I have stock monifolds, back to dual Flowmaster. One O2 on the pass side below the starter. I'm no good with computer, or computer controlled data, but I'm fairly certain when I sent my ecm chip in to wipe out the vats that they deleted all emission data as well, does that include the O2? When I unplug it, I have the same issues. Wish I was more educated in the computer stuff guys. Please teach away.

Alaskanbadfish

I haven't thought of an exhaust leak being the issue until now, thank you. My exhaust was very strong in the cab while driving before this episode. What is the best way to check for a leak?

hrcslam

A popping sound and/or black soot from where a leak would be. An exhaust leak before or near (even after) the O2s will cause false "lean" readings. This will make the ECU add fuel. This makes the fuel smell stronger, and can cause erratic engine behavior.

timopajala

I bought ac delco from rock auto and that was plug and play. Have you checked vacuum leaks etc? I had vacuum leak in my car but there was no surging so i don´t think that its the case here. How is the fuel pressure? How old fuel pressure regulator?

Alaskanbadfish

Fuel pressure regulator hasn't been replaced at all that I know of. Fuel pressure is around 45psi.

The throttle body is super loud when it's acting up, hissing/sucking air very loud. I'm going to be concentrating today on exhaust leaks, I know there's something there because the smell of very rich exhaust is super strong in the cab when driving.

Alaskanbadfish

Went through and really torqued all the manifold bolts, they were all fairly tight, tightened up a bit. Tightened up the connections after the manifolds. Didn't notice any visible black marks of a bad leak. Unhooked the battery and will wait ten min to try and restart. Also inspected all vacuum lines/connections, all look fine. Crossing fingers.

Alaskanbadfish

Restarted, and other than two small fluctuations for just a split second each, everything leveled out, ran up to operating temp and all appears normal. There is still a small leak above the O2 where the exhaust mounts to the manifold, I'll let it cool and work on that (some strange brass type nut that is getting smashed when I tighten it) I'll replace that and am hoping it's gtg. Unhooking the computer/battery might have helped too. I'll keep yall posted.

lt1-xjs

There should be 2 02's on the engine, one on each side.

Alaskanbadfish

This is all aftermarket, it's in a jeep. Just one O2 right near the starter.

So after test 2, all is perfect, I'll go ahead and sign off this threat, thanks for the help. Who'd of thunk a small exhaust leak could cause me such a pita...Thanks everyone.

shbox

Only one O2 sensor? How can that be working properly when your computer is looking for one for each bank? The bank without the O2 sensor must be fueling very oddly. This is where a data log would show you just what is going on.

Alaskanbadfish

Yes just one O2, the shop that fabed my exhaust they weren't going to do any. I've often wondered this myself, but for the most part it's been a reliable daily driver with minimum issues.

hrcslam

I think itd be a good idea to get that second o2 sensor installed. Right now your engine has no idea what each bank is doing (assuming that 1 o2 is talking to both banks of the pcm). You could be lean on one bank and rich on the other. This can be expensive if not repaired. But you have no idea of it is or not without that 2nd o2.

What mods does the engine have?