Thread: 93 v8 works hard for a few seconds, then runs on 4 cylindrers.. help!

Is the running on 4 cylinders educated guesswork or an actual count? Does the engine running return to 8 cylinders abruptly or gradually?

The fuel pressure regulator has a short vacuum hose off its back end that goes to manifold vacuum. Pull off the hose and look for softening and signs of petrol in the vacuum hose.

The injectors run in two banks of four, one bank cutting out may indicate a problem with the ECU, or the harness plug at the ECU.

Have you cleaned the air flow meter (MAF) ? You need a spray can if contact cleaner or MAF cleaner and spray into the air cleaner side, carefully. There is a small hole off to one side that contains the sensing element.

Remove your spark plugs. If the engine is running super rich they will be black to dark brown.

Originally Posted by bee utey

Is the running on 4 cylinders educated guesswork or an actual count? Does the engine running return to 8 cylinders abruptly or gradually?

The fuel pressure regulator has a short vacuum hose off its back end that goes to manifold vacuum. Pull off the hose and look for softening and signs of petrol in the vacuum hose.

The injectors run in two banks of four, one bank cutting out may indicate a problem with the ECU, or the harness plug at the ECU.

Have you cleaned the air flow meter (MAF) ? You need a spray can if contact cleaner or MAF cleaner and spray into the air cleaner side, carefully. There is a small hole off to one side that contains the sensing element.

Remove your spark plugs. If the engine is running super rich they will be black to dark brown.

hi thanks for the reply.
it is an educated guess that its running on 4, when it runs on 4, after 2 seconds it runs on 6 then 2 seconds later it runs on 8. (you can hear it)

we just replaced the hose you were talking about and no change..

it might be helpful if i mention that when we start the car it revs excessively high when in park or neutral, then after a while it goes back to normal. does this help?

it sounds more like a case of sticking valves to me, if you go and run it hard up a hill, then immediately pull over and let it idle, does it slowly regain all cylinders again?

jc

may also be injector looming failing at the bends, but I'll back the valves sticking open a little as my primary suspect.

to confirm try this.

with the engine idling normally remove the connectors to some of the injectors and with experimentation you will be able to replicate the "firing beat" of the engine when its misfiring. What you will notice is that when its doing this against full compression the engine will rock more and the "beat" will be harsher. When its happening naturally it will sound "smoother" and any vibration you feel will feel less due to the missing compression stroke.

IF that fits then its the valves

okay so thanks for the replys, we went out today and wired up a CO2 sensor, what we found is under heavy acceleration all was fine reading around 14, but when the misfiring starts to happen it leans WAY out.. does this help?

the misfiing makes the whole car rock and is very uneven. so its basically narrowed down to a fuel issue.