The car idles horribly after it warms up. When I let off the gas to coast it backfires and misses, then when I come to a stop it will stall. When I put the engine under any load the car runs fine not even a noticeable loss in power. I performed a search and already used propane to check for vacuum leaks, which was what I first suspected and after research on this site. I recently changed the fuel pump to a walboro 255 pump so when I checked the fuel pressure it was normal. The car did this a few weeks ago and I found a broken vacuum elbow for the pcv valve. I replaced the elbow and all was good for a while. Now Its doing it again, so I checked the elbow again and replaced the pcv valve with a new one. The optispark only has about 10k miles on it and plugs and wires are new. The o2s have about 10k miles on them as well. this is my dd and its really annoying me. please help

 

All the plugs look good? New fuel filter with the pump change?

 

i did pull one plug and it looked fine, the fuel filter wasnt replaced then but has been replaced in the last few years. The fuel pressure is fine and the problem is worse at idle and when you let off the gas. Could it still be the filter? It seems to be vacuum but I cannot find it or hear it.

 

Codes? IAC...datalog it.

 

only codes are for the rear o2 sensors, egr that was deleted and air injection also deleted. I have the GL1 gopoint cable and App for my Iphone for scanning, It does show some real time data such as o2 voltages, mass air readings, map sensor readings, and fuel trims. Both banks of o2s seem very responsive and show even readings. I dont know how to get IAC counts but I did check the resistance using the sticky to shoebox's website.

 

take the IAC valve off and check it for buildup. i cleaned mine and it cleared right up.

 

I did clean it a few weeks ago when it first started to act up and it did not help, but maybe it is starting to fail? When I hook it up to my scanner the map sensor is reading 8"mercury.

 

its cheap enough to just guess and check. you should have a much higher vacuum at idle. hook a gauge up and see what it really says

 

It runs like crap after the car goes into closed loop which makes me think that it's a fuel injection problem caused by a bad sensor (MAF or precat O2). Unplug the MAF and warm it up. I had a similar problem when my driver side O2 went bad. The car would run fine on a cold start in open loop but it would begin to misfire, surge, and backfire when the PCM went closed and used O2 data in adjusting the fuel trims.

Are there any mods to the car?

 

i did unplug the mass air sensor but did the same thing after it warmed up. mods are LTs, full exhaust and cai. egr and air has been deleted and have been for years with correct block off plates and gaskets

 

anyone know the normal map sensor reading on an lt1 at idle?

 

I always got around 8~10"Hg.

 

well i am getting 8" at idle so i should be good there. I do not know what to do now..........thanks for the info!

 

Exhaust leak around the headers/y-pipe? What brand O2 sensors?

 

no real noticeable exhaust leak, o2 sensors are boush (sp?) and are not too old. dumi bought them before I knew they were crap. dumusing my tuner to monitor the sensors and they are changing voltage constanly and evenly like they should so I dont suspect the o2s. dumwhat about map iac and tps sensors? I unplugged the maf but its doing the exact same thing. dumruns fine when cold.

 

well I hooked up a vacuum gauge and got 20"hg at idle so I think a vacuum leak is out of the question. The map sensor reads 20-30"hg when the engine is off but ignition on. When I fired the engine up the map sensor went to 9"hg, is this normal? Is there an offset with the map sensor? Another thing I noticed is that the short term fuel trim does not seem to be doing anything at all but the long term trim seems fine. I do not know what to suspect now. What are normal maf sensor readings at idle? I did unplug it and tested it while warm but did the same thing, should I unplug it while cold and try it? Sorry for such long posts but trying to give the most details possible!

 

Ken,

When you first start your car, the MAP sensor reports standard atmospheric/barometric pressure -- usually around 29-30"Hg, depending on where you live. That's just the pressure of the air in the atmosphere pressing down on you, and thus, pressing inside your intake manifold as well. (With the engine off, there can be no vacuum -- pressures are in equilibrium inside and outside the intake manifold.)

When you start the engine, then you have your normal idle MAP reading (around 9"Hg). Quick Tip: Instead of pulling out the mechanical vacuum gauge, you can just subtract the idle MAP reading from the earlier barometric reading with the engine off. The result of that calculation yields your engine's vacuum reading. (29"Hg - 9"Hg = 20"Hg vacuum. The same as your vacuum gauge!)

Incidentally, when you go wide-open-throttle, the MAP reading (of 8~9"Hg) will rise until it matches the earlier BAR pressure of 29"Hg. In other words, engines produce vacuum at idle, which dissipates as you approach WOT. Things like your fuel pressure regulator operate on this principle, using vacuum to reduce fuel pressure at idle, but not at WOT.

Anyway, I haven't actually read this whole thread yet -- just popping in with quick answers. I'll give it a read and post if I have any wild thoughts.

 

Okay, few thoughts to keep the ball rolling:

-----------------------------------
(1) For the engine to run rough *only* when warm, that typically either indicates an electrical component failure (heat and electronics don't play well together), or it's a closed-loop fueling problem. The PCM only goes into closed-loop after it warms up a little.

(2) Yes, supposedly Bosch sensors / spark plugs / wires wreak havok on a lot of cars. A local shop told me that the vast majority of the problems with their customer's cars end up being related to Bosch components. So, I certainly wouldn't rule that out yet.


(3) You keep saying that things checked out fine, but you're not listing specifically what they were:
(4) You said you cleaned your IAC. Sometimes, if you accidentally press on the IAC's pintle, you can screw it up and it wont work properly. The fact that it bogs when you let off the accelerator sounds like a possible IAC issue, but it's hard to say at this point. There's an IAC reset procedure, but I don't remember the specifics -- something like stepping on the gas pedal all the way while starting, let it idle for 5 seconds, shut off the engine, then restart normally. Also, you could try opening up the throttle blades a little to see if it idles a little stronger. Throttle stop screw is an allen-bolt on the top-driver's-side (rear) of the throttle body -- kinda recessed. If you manually open the throttle blades with the car off, you should be able to see the screw shank holding the throttle mechanism open on the front-top-side of the throttle body. If that helps, your IAC might be buggy.


(5) For the time being, let's rule out vacuum leaks. The sensor readings will likely let us know if that's the case. MAP readings look normal. If you have lean long-term fuel trim numbers, then I'd consider looking into it more. Could be intake manifold gaskets, etc.


(6) It's been a long while since I was stock, but I think my stock-ish MAF readings were around 10gps. What are you reading?


I could probably keep going, but I'll let you chew on that for now. Sometimes stuff like this is like trying to find a needle in a cliché. I've probably forgotten something simple and important...

 

Not sure if it's been mentioned, but unplug the O2 sensors, drive around and see what happens.

 

If u do that wont the car stay in open loop?