I am still trying to figure out how to get an 84 2.5L to run. The engine only has about 35k miles on it and was just great for 5-6years. Then one day it just dies, restart it and it just keeps trying to die but will surge up and down on its idle when in park. I pulled a code 13, 34 and 35 last week. Since then I have replaced; spark-plugs & wires, dist. cap ^& rotor, O2 sensor, MAP sensor, TPS, IAC (several types, but will only barely run on one from 87 Corvette), Completely disasembled throttle body to rebuild/clean replacing injector and Fuel press. reg., cleaned ports to EGR and vacuum checked egr, cleaned air cleaner(K&N). I checked fuel pressure got 13.5to13.8. Still will not run right.
When cold it doesn't want to start but if i keep trying while pumping the pedal it will eventually start but i must keep pumping pedal to get it to surge up and down untill finally surges up to around 3000rpm and warms then it will idle and surge on its own to about 800rpm to almost dying then upagain it will some times die while doing this.
I give up.. I can't think of any-thing else to check. my auto-xray still says that the O2 is lean but i can smell fuell in exhaust. Since that connects and reads i would beleive the ecm is ok.
Any ideas? Any-body else ever have this happen?

Check the fuel pressure and exhaust for plugged cat.

My fuel pressure is just under 14psi. It is hard to read on my higher gauge(75psi gauge). I have been wondering about the cat but still feel exhaust coming out rear. The weird part is that i don't feel much exhaust while the idle surges up but feel it durring the stumble as it goes up &down...I was thinking that it might be just the distance that the exhaust flows thru the pipe that the second or so between surging up and stumbling causes the delay in my feeling it at the back???? is there any other way to test the cat? Or does this meen a clogged cat?

Todd, Bang on the cat to see if you hear any loose parts rattling around in there. A few things for eratic or loping .
1. vac leak
2.leaky egr vavle or plugged pcv vavle
3. fuel pump not delivering suffiecient fuel
4. leaking head gasket ( probally not)
5. cam shaft lobes worn (probally not)

Ok, Now i have changed the EGR and PCV(no change!)....I am going to drop gas tank tommorrow to switch fuel-pump, strainer and pulsator dampner...
What i really want to know is if there is a way to test the catalytic converter with-out removing the pipes? I am still no certain of it.
After every-thing else that has been done i figgure an oil change and it shouldn't need any-thing for another year! Better not, or i may burn it to the ground out of frustration.

My 2.5 did the same thing...But I was running without a Cat....I never did figure out the problem ...but it seemed fuel related.

Another thing to check on the 84 are the two fuse links at the starter, they supply almost all the power to the car and don't last long with the heat from the engine, exhaust and CAT.

If they are failing it could cause numerous problems, when mine went the car died.

A visual inspection should show if they are cooked as you should see bare and/or broken wires where they connect to the starter with the positive battery cable.

I had them go bad twice on my 84 and finally extended the wires, added new fuse links and connected them to the positive battery terminal as seen in the attached pic.

I'm sure you've done this but you didn't list it above... did you change the fuel filter? How bad off are the hoses from the gas tank? Does the exhaust manifold glow noticeably at night after running a few minutes?

This is it...or don't buy me a beer if it isn't.

Coolant Temperature Sensor*. Located a few inches down from the coolant tube cap on the engine. This sucker feeds vital info to the ECM. If a wire or connector breaks, or the sensor itself fails, the symptoms are like you describe. Basically, the ECM "thinks" the engine is VERY cold and a rich mix gets dialed in...or something like that...it is well past 2am and my mind is switching off.

Try disconnecting it in your car. If you notice no difference, you'll know it was failing.

For my 86 L4, Borg Warner had a part the with the proper connector. Other brands may require tinkering/soldering.

Good luck!
Beno

*There is another temperature sensor that feeds the guages. I believe it is called "coolant temp. switch and sender."

I had similar issues that I tracked down to a faulty EGR circuit. Replacing the valve may not do a thing. The throttle body is responsible for signaling the ERG to open or close with a ported vacuum signal sent to the EGR. If there is a problem with the metering, the EGR gets activated at times that it shouldn't.... the result is erratic idle, intermittant stumbles, and in gereral can cause some really crappy drivability issues. The easist way to see if this might be the case is to remove the vacuum line to the EGR and plug it. If your problem goes away, you now know the problem.

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Competition Yellow 84 2.5 Bone stock right down to electrical gremlins

Ok, For those with similar issues. This problem was solved by replacement of fuel pump...But while out i found that the real problem is the cheep POS hose (Short peice attached to pump itselft) that had been installed previously. The failure of this hose caused a delamination type condition that appears to partly close as pressure builds then opens opens again for some reason. My higher reading gauge would not show the slight up and down of the pressure!
Now how do i fix the fuel gauge sending unit since i seem to have killed that while changing the pump? I had a problem with the float falling off the unit durring both removal and install. Is there a reason that it does not seem to be very solidly attached? The tank hole is impossibly small for the R&R of the assembly. Is that why the float comes off so easily? And is there some trick to getting it back in that i should know?
Thanks for the help.

This will help with the sender unit, as far as the float issue, it does fit back in there, you just have to wangle it, if it fell off , you may want to look in your tank while its out, for ?? also a good time to clean the tank.
http://www.fieropride.com/tech/gas_guage.htm