Hey Guys,

The car is a 1990 IROC, 5 spd, 305 TPI, stock with K&N's, hi-flows, and Flowmaster.

The car has been running fantastic, especially as the weather cools off here - it loves the cold air! But ...

I drove it hard and parked it two nights ago. It ran perfectly, no loss of power, hesitation, or issues through the rev-range. The next morning, it took awhile to start (not uncommon when it gets cold out), and I immediately noticed a large hesitation as I let out the clutch in first gear. The car lost power like it stalled, started back up, stalled, started back up, etc. It created this fairly violent bucking and driveline lash. (FWIW, I kept throttle position constant). It smoothed out and then did it again, except at part throttle and second gear. It smoothed out, but then as I drove up into 2500 - 4000 range it began to lose power, like I gradually put the emergency brake on.

It's been doing this for two days now. Bucking, power loss throughout the rev-range, and occaisional lengthy warm starts. It's like the car is choked or something, or it's not getting enough fuel. There are times when it accelerates like normal (5% of the time), for one gear the car runs like it should, the other 95% it feels very bad. If I roll into the throttle at low speed in first or second gear the car hesitates and feels like it will die. If I'm in second and run the car up from 2000 - 5500 it feels almost normal till about 3000, than just loses power from there. The exhause even sounds different, less throaty, like its a bit muted. I don't get it.

For example, I can hold 1/4 throttle in first gear at 5 MPH and as it rolls through to the redline the RPM will flutter, the car will lose power and cut out. If I transition to full throttle suddenly, the car feels like it's got half of the HP/TQ on the top end of the rev-range, where it typically has the most power.

The times that it happens seem completely random; I haven't noticed a trend of cold motor, weather, throttle position/speed, etc.

There is no CEL, no codes. I just put a tank of fresh premium in it with no change. When the car is in neutral, it idles fine 95% of the time; it hasn't died or anything while at idle. Also, I can hold the throttle and slowly bring the revs up from 700 - 4500 and while it sounds a bit muted, the car does not hesitate while spinning freely in neutral. It seems to only buck and hesitate under load.

Any ideas??? I've tried to be as descriptive as possible. The problem seemed to occur overnight, with a sudden decline in performance, not a gradual one.

Solutions/Help would be greatly appreciated! If it's possible, I'd like to fix this myself!

Thanks,

Clint

EDIT: And before anyone asks, I searched! And found nothing of help resembling my symptoms.

 

Welcome Aboard!

Common causes for stalling under load and lack of power are weak ignition, low fuel pressure, restricted exhaust, and poor compression. Have you checked for any of those possibilities?

 

Thanks for the welcome!

Have not checked fuel pressure or weak ignition. Unless the cat is clogged I cannot imagine the exhaust being restricted; how could I tell if that is the issue? And doing a compression test would be one of my last options. I'm betting it's a fuel problem, but would something like that occur overnight?

 

Sounds to me like the pump is taking a dump on you! Any idea how old it is? Did the pump sound change at all? Get louder than normal? If it is the pump put a GM pump back in it. Aftermarket pumps are junk!!!! Its too much work to have to redo it a year or less from now!

 

Bottom line, you cant tell over the Internet. Even if you describe the symptoms in detail, the car needs to be tested to see where the problem is. After recently experiencing a similar problem I would break it up like this. Its either a mechanical problem with the engine, or its a problem with one of the parts that support the engine. So first thing I would do is a compression/leakdown test to make sure its not a mechanical problem. Then once you prove that out, move on to the parts that support the engine. I would break that up into 5 things, fuel, air, spark, vacuum, and ECM. If the ECM shows no trouble codes then good chance its not the ECM or one of the sensors (not always true, but good chance). So from there I would test fuel, air, spark, and vacuum. To me vacuum, fuel, and air are easy to test so I do those first. Run a vaccum test, really easy to see if there is a vacuum problem. Then remove the air filter to ensure no restriction to the throttlebody, that takes care of air. Fuel is also an easy test at the fuel rail, where to go from there depends on the results. IMO spark is the hardest thing to test, this would include the ignition system. I spent the last 5 months troubleshooting what I thought was an ignition and possibly ECM problem. After replacing the TPS, IAC, EST, Distributor, Knock Sensor, ECM, Plugs, Wires, Cap, Rotor I still did not fin the problem. Ran a compression test and results indicated my head gasket is blown (even though I have no signs of a blown headgasket like oil in the coolant vice versa). So this is why I say do the compression/leakdown test first. Dont end up like me and spend months and alot of $$ barking up the wrong tree.

 

All,

Happy New Year! Thank you for all of the replies; the problem was the fuel pump and relay module. Had both replaced and the car runs very well now.

Thanks for the advice and help. Very much appreciated.