90Iroc350

I have no idea where to start. Last fall i purchased a 1990 iroc. It has the 5.7l 350 automatic. The guy said it sat for a while and he had a mechanic try and get it running good but he couldnt figure it out. He changed the distributor cap and gave up. I paid $1500 canadian which was a steal i knew from the sound of the car they mixed up a couple spark plugs. Sure enough i brought it home and found the two they mixed up and switched them. It ran perfect (it was late fall so it was pretty cold outside). First to second gear the tires would spin so i put a permit on it and drove it until i could get it safetied. I noticed on the highway it was amazing but in the city the rpms would jump from 1100 to 1600ish and just keep jumping back and forth. I ended up just parking it for the winter because i also gave up and could not solve the issue.

I pulled it out a few weeks ago and read plenty of forums which describe my issue or close to it. I noticed now that it had low end power but at about 3K rpms it would have no power and as it warmed up it just got worse. In fact no matter how hard i pushed the petal it wouldnt even pass 4K. I tried timing it multiple times just turning the distributor slightly both ways not using a gun. I can get it so that the RPMS dont jump but then it has no power at all and takes around 30 seconds to reach 50kms/h. People said vacuum leak but that doesnt seem right to me and as far as i can tell its not the cause.

I took out the complete distributor and compared it to another one and mine was missing a ring around it so i changed the complete distributor (except the cap since he changed it). it seemed to be running alright the rpms didnt jump as much but it could be the way it was timed. But still no high end power and when it would warm up (around 105) it would almost die or completely die and then not start again. I would have to let it sit for around 30 minutes before it would start which i was ok with because at least i could get home. Tonight i took it for a drive when there was no traffic to do some testing but i got stuck behind a train and it reached 105 and stalled. Once again it wouldnt start so i pushed it to a parking lot and let it cool down for 50 minutes but it wouldnt start.

Things i have tried/replaced:
-Throttle Body (the Position sensor and i believe its the iac thats on it)
-Spark plugs but not wires. Wire 1 has a cut in it but i dont believe it is whats causing my issue.
-If the previous owner changed the distributor cap then the complete distributor is changed.
-Exhaust gasket
-Manifold Gasket

Side note - the vac system is still in but i have it sort of bypassed with a resistor it sometimes works sometimes doesnt i just need to play with it a bit and she starts. The polution pump is still hooked up and sometimes makes a really annoying humming sound. To me it almost seems like it has a racing cam the way it shakes and sounds. Another possible thing that i have not checked it possibly an aftermarket chip for the ecm

-Here is a quick video showing the rpm jumping
Sorry for such a big post i just really want this fixed and its driving me crazy that i cant figure it out. If anyone has any ideas or even has a temporary fix for this that would be great. Thanks in advance.

Tootie Pang

iTrader: (2)

I did not see that you read any codes. You should do this first. Make sure the check engine light bulb works.

If you replaced the throttle body with a new IAC and TPS and it made no difference then I would look elsewhere.

From the video, it seems to be idling smoothly so I would not suspect a major vacuum leak.
You changed the distributor which means you changed the pickup coil and module.

At 105 celsius, that's about 221 F which is a normal operating temp and the ECM should go into "closed loop" mode, and start using the O2 sensors. It is possible your O2 sensors are gone. There are very clear codes for the sensors.

When you say vac system, do you mean VATS?

90Iroc350

Sorry i went camping but yes i mean the VATS system. I have not thought about the O2 sensors but i dont see why that would prevent it from starting after cooling down. I would think its the fuel pump. The way it would stop running after warming up and wouldnt start. Im not sure if when a fuel pump is going if it can cause the rpms too jump or if thats another issue.

Also yes the check engine light works but its not on.

battman

iTrader: (2)

Sounds like a fuel delivery problem..start with checking fuel pressure, if pressure is good then check fuel injectors.

GeneralDisorder

iTrader: (1)

Check what the computer thinks the coolant temp is.

You may need to check other sensor data. I suggest a live data scan tool such as the ALDL Bluetooth interface for your phone, etc.

Rick

Tootie Pang

iTrader: (2)

When a fuel pump begins to fail, it develops "dead spots" when it stops. If it stops spinning and lands on a dead spot,you may need to knock the tank with rubber mallet or something to get it do go again. These dead spots will spread until your pump stops altogether.

RedLeader289

iTrader: (2)

Ignition Coil.

You said you replaced the Distributor but left the cap, did you replace the ignition coil?

When they start to go they can't tolerate heat, the engine will die and not start until it's cooled down, only to run for a little bit until it reaches --- temperature and die again.

You can bench test them with a multimeter, Also check your ICM (ignition control module) because if the coil goes they are usually quick to follow (or have shorted out and therefore caused the coil to fail). A faulty ICM can also cause RPM issues.

I've made a few threads on this.

southbay08

check the resistance on your injectors when they are hot. The stock multecs are notorious for coil failure especially when they are hot.

vette9190

iTrader: (2)

Check The injectors. I had a problem like this. when the engine was cold and running in open loop it was fine. Once it reached operating temps and went into closed loop she would stall and sputter and not restart. It turned out the 20 year old injectors just got to weak and/or clogged up. I replaced them with new ones and the problem went away.

90Iroc350

So i tested the fuel pressure and it said roughly 10 pounds. Does anyone know what the pressure should be at?

GeneralDisorder

iTrader: (1)

Regulated pressure with engine off should be 44.5 psi or so. Give or take a few psi won't matter.

10 will not get the job done. Check the regulator - might be bad.

GD

Tootie Pang

iTrader: (2)

Yup sounds like the culprit if it never gets above 10 psi.

90Iroc350

Before i tested the fuel pressure the car was running for about 5 minutes but it stalled and wouldnt start again which is why i tested it. Now i tested it the next morning after sitting all night and it was at 40psi. Is it possible the fuel pump is going but just isnt dead yet? can the regulator cause this?

GeneralDisorder

iTrader: (1)

Apparently this is common for the stock fuel pumps to fail like this. There's another thread going where that seems to be the consensus. Get a more modern turbine style pump.

GD

90Iroc350

So i changed the fuel pump with one from an older truck and now the car starts fine but there wasnt much gas so i didnt run it long. Only issue is im only getting around 15-20 psi so i would think i got the wrong pump. Ill most likely buy a new one in the next couple of days and test it out ill reply back here if it works.

Tuned Performance

iTrader: (92)

If it's from a tbi truck that explains the low fuel pressure. Use a delco sp241 pump.