Need Advice: Miss, low rpm stumbling

[EWest]

Hey guys, I am new to the forum and relatively new to the ZR-1 scene. I was hoping for some help with my '90. I have seen quite a few posts about engine hesitation etc., but none exactly like mine, so here goes:

There is a pretty severe miss during cold idling, and sometimes less severe while hot. At low rpm acceleration there is a severe stumble / hesitation up until about 3000rpm, when she takes off like nothing is wrong. WOT at low rpm usually causes severe stumbling and backfire through the plenum, but oddly on occasion she accelerates fine. The less throttle you give, the less severe the stumble seems. It seems that the lower the rpm the worse it is, especially in higher gears.

Idle is at about 800 rpm.

I replaced the fuel injectors with the rebuilt fic ones, and had to pull the plenum again to fix a vacuum leak in the intake. No change. Plugs and wires are new. I'm kicking myself for not replacing the coils while I was in there, but I did the "pull the plug wire and check the arc" trick and all seemed to have good spark.

I realize there are sensors etc. that I should test, but where should I start?

Thanks.

[Tyler Townsley]

Check the fuel pressure. Usually you can use a tester at tool loan program at some auto parts places.

Tyler

[scottfab]

I agree fuel pressure is a good place to start. One thing these cars don't like is to get into hard long ~1G cornering (exit and on ramps) with <1/4 tank. They can suck air even with the little tub that surrounds the pumps.
Sucking are = pump death.
Static test of the pressure should be > 50psi and hold fairly well.

You should also tell us how heavily modified the car is.
When things fail some just choose to rip things out.
Do you have secondary actuators?
Stock chip?
exhaust?
Gas is old? (car been sitting)

[EWest]

Car is completely stock to my knowledge including exhaust. The car had been sitting off and on for some time, but I've cycled through several tanks of gas at this point.

[scottfab]

Originally Posted by EWest

Car is completely stock to my knowledge including exhaust. The car had been sitting off and on for some time, but I've cycled through several tanks of gas at this point.

Let us know what the results are of the pressure test.

[EWest]

OK I did the static fuel pressure test, and the pressure was 45 psi with the test connector jumped. I bled the line, took off the jumper and started the engine - back to 45 psi and held. Thought that was odd as I figured the primary pump would be at fault due to the symptoms. Both pumps are operational as they read 9.74 A while doing the test.

Any thoughts?

[sammy]

did you try the test that's used in the fsm . attach the gauge to the shrader valve and just turn the key to on for 2 sec then off and see if the pressure holds for about 5 min . before going down ?????? there are some threads that tell you how to test the coils without pulling the plenum .. do you have a scan tool to ck the operation of some of the other sensors ?if not see if a buddy has one .it could save you alot of time and money .

[tomtom72]

Cold idle = really bad miss? Then I would check the IAC, temp signal to the ECM from the discrete sensor, also IAT & TPS sensors. I'd look at the "cold start" sensors and what signal they should be sending to the ECM.

Warm miss = I'd still look at the TPS and coolant temp sensor & IAT and see that the IAC is at the correct # of counts. You said the ignition system is good? Also, the rebuilt injectors are an outside possibility, very outside.

Fuel pressure = I have a sense that if 45 psi is the best reading you got then the pumps are checking okay, but they're at the lower end of the range the FSM has for good pumps. I doubt that this is the reason for the miss. Fuel filter is good, right?

You are sure the MAP hose and MAP are good? The MAP & hose can become flooded with the blow by oil and that causes the sensor to give bad signals to the ECM. Also, make sure that the vacuum hose is the correct type, a hard line and not standard soft rubber.

If you have a scanner you can look at all these parameters from cold start thru warm up to normal operating temp and compare your readings to the FSM's numbers. You didn't mention a SES light, so I figure that there are no DTC's?

Oh forgot this. When you put the plenum back on you are sure that none of the injector harness wires got pinched. Another member had this issue & it drove him crazy as it yielded a miss. It's kind of cramped under there as you know!


Tom

[XfireZ51]

When was the last time fuel filter was replaced? Fuel pump "socks"?

[EWest]

Noticed something today that I had not noticed before, vacuum pump cycles on briefly about every 15 seconds with key in ACC position. Looks like I do in fact have some sort of vacuum leak, but would that cause this severe of a miss and stumble?

Fuel filter replaced recently, don't know about the fuel pump "socks".

Forgot about this, but the only time I get a SES light is if it backfires. It has been a while, so would it still be stored?

I'll check the MAP & hose for oil.

Don't know anyone with a scanner to check the other sensors, what type do I need?