HELP! suicidal '90 Renix 4.0

[A-man930]

I don't have much time, but in short my engine will start fine, but will shortly go into an irradic idle (sometimes down to 200rmp or so) and will occasionally kill itself. I can feel it while in gear as well. So far I'm thinking its spark related... it seems like i'm losing cylinders. Common issues? Guesses?

[Eagle]

How much time before it goes loco?

 

Initial guess --> oxygen sensor bad. Explanation: On a cold start, the engine by-passes most sensors and runs in "open loop mode," in which it pretty much ignores sensor inputs and uses a pre-programmed, slightly rich fuel map to run. Once the temp sensor sees that the engine is up to the operating range (around 180 degrees, minimum) it switches to "closed loop mode" operation, in which it reads the O2 sensor, temp sensor, MAP sensor, TPS, etc and controls the fuel mix and ignition timing accordingly.

 

If it runs okay cold but goes wacky once it gets into closed loop mode, there's likely a bad sensor. The task then is to identify which one.

[A-man930]

it does it sometimes as soon as it settles into an idle, open or closed loop.

[SomeNew Name]

YOu know what mine does the same thing.. feels like it's being loaded eh?

I had replaced my o2 sensor a while ago, and tested it, and every other sensor on the truck but still no relief. Also I'm sure the truck dies when you give it a shot of gas, when it starts to idle in the 200rpm range, the shot of gas jumps the rpm up and then dies or drops dramitically right?

 

if so, than you have the same issue as me... if you figure it out Let me know please!

[88XJSport]

I say check your fuel pressure to make sure it isnt slooowly dropping below sufficient psi (~30)

[A-man930]

Well I went ahead and took it to a shop and they said I had an O2 sensor reading over 1 volt (something was shorting out??) which makes the computer go nuts regardless wether its in open or closed loop evidently. I'm going to go ahead and try it and see what happens. As for the fuel pressure slowly dropping, wouldn't that affect the whole time the truck is running? Remember, this is only happening intermittently... not all the time.

 

and yes wakka_round, you're describing the same thing... which makes me hesitant to start replacing stuff

[88XJSport]

I was under the impression it happened at the same time. I read your post wrong. My bad.

[SomeNew Name]

well I've haven't been throwing money at it either, but I've ben trying really cheap stuff...

like redoing all my vac lines, the hoses were pourous as hell.. no difference,

I pulled my injectors the other day,, and they were in some rough shape. I can't afford new ones so I'm going to have to figure out how to get them cleaned by myself... never cleaned and injector before.

 

 

well I have another question for ya.. maybe you have the same thing too.. do you find the battery dead quite often? after letting the truck sit for a few days?

[A-man930]

Turned out that the problem was a bad o2 sensor, she runs just as before... nice cheap fix (except for the stupid diagonstic fee :mad: )

[SomeNew Name]

well that's rather weird then cause in my case the o2 sensor is brand new out of the box.. the truck hasn't been driven on the street since I installed it... and there was no difference in how it ran before vs after..