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Old 11-06-2013, 03:57 PM   #1
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94 z28 hesitation and rattle upon acceleration?

What could cause a 94 z28 to hesitate upon acceleration when this happens engine makes a rattling noise. No signs of water in oil, heat works great, doesn't run hot. Does not lose water in radiator. Replaced spark plugs with Taylor wires 11/05/13. If I use Lucas fuel treatment seems to stop until next tank or so but then starts again. The car also loses power. Any ideas?
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Old 11-06-2013, 04:33 PM   #2
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Re: 94 z28 hesitation and rattle upon acceleration?

Are you using high quality gasoline ?? may be spark knock ?
did you gap the plugs properly ?
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Old 11-06-2013, 05:06 PM   #3
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Re: 94 z28 hesitation and rattle upon acceleration?

What grade gasoline are you using?

Check for oil in the vent line from the throttle body to the passenger valve cover. If there is excessive piston ring blowby, the line will flow in the wrong direction, to vent pressure from the crankcase. It entrains oil droplets, they flow into the chamber on top of the throttle body, then drip into the intake. Oil will significantly reduce the effective octane of the gasoline.

It would take an awful lot of spark knock for you to hear it. The knock sensor should pick it up before you can hear it, and retard the timing up to 15* to eliminate the knock.

Another possibility - check the 90* rubber inlet elbow for tears or the bottom edge not being fully on the throttle body. If air bypasses the MAF sensor when you hit the throttle, the PCM will not add enough fuel, and it will run lean, possibly causing knock.
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94 Formula A3+1: 381ci all-forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams custom solid roller / TH400 + GearVendors OD / Strange 12-bolt - 4.11's / 300-shot N2O / Spohn suspension / 6-pt roll bar with integrated SFC's / AutoMeter instrumentation / MoTeC M48Pro engine management system / a few other odds 'n ends. 800HP/800LB-FT at the flywheel.
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Old 11-06-2013, 05:10 PM   #4
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Re: 94 z28 hesitation and rattle upon acceleration?

I see you already started the same topic on the LT1 Based Engine Tech forum on 10/30. Please do not post a topic more than once. For example, parkers_Z28 already responded to your earlier post, but you didn't answer his questions in that thread. Now he's asked you the same questions again.

94 z28 running rough & rattling

Keep all the give and take in a single thread.

Thank you.
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Old 11-11-2013, 03:48 PM   #5
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Re: 94 z28 hesitation and rattle upon acceleration?

Thanks for the input I'll check these issues, my apologies for the double posting, I am a 1st timer on this site.
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Old 11-13-2013, 11:23 AM   #6
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Re: 94 z28 hesitation and rattle upon acceleration?

I have checked these issues, I have been using 93 octane gas, I checked spark plug gap with a gap tool. All gaps were .050, which is what manufactor specs recommend, will a bad cat. Cause this problem or bad fuel filter?
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Old 11-13-2013, 03:26 PM   #7
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Re: 94 z28 hesitation and rattle upon acceleration?

Weak fuel pump, or clogged fuel filter could cause the hesitation. Check the pressure, out on the highway, under load - full throttle/5,000+ RPM. Should hold at least 40psi.

Plugged cat will usually cause it to reach an RPM limit and then not go any faster, as the plugged cat restricts exhaust flow. If it's the cat making the rallting noise, and not the engine, that might be indicating a problem.
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Old 11-18-2013, 07:07 PM   #8
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Re: 94 z28 hesitation and rattle upon acceleration?

I did a block test today 11/18 using the block tester you get at autozone. It turned from blue to yellow, so it is possibly a bad head gasket, but there is no water in the oil, has the same amount of coolant in it as the day I bought the car six months ago, smokes when 1st started in the morning but soon stops. Heat blows hot and doesn't run hot. No signs of blown head gasket. Maybe a bad intake gasket? Idk???
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Old 11-18-2013, 07:10 PM   #9
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Re: 94 z28 hesitation and rattle upon acceleration?

Also, it does seem to rev high under high acceleration, I think I'm gonna have the cat cut out just to see how it runs without it.
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Old 11-18-2013, 08:19 PM   #10
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Re: 94 z28 hesitation and rattle upon acceleration?

The LT1 intake is dry - no coolant. Not like the Gen 1 SBC.
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Old 11-19-2013, 11:58 AM   #11
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Re: 94 z28 hesitation and rattle upon acceleration?

Ok so the coolant flows from the head to the block so with a bad head gasket would I still have water in the oil or would the water be burnt out by the spark plugs, also how accurate is the the block tester? I did the same test on my wife's honda which does show signs of a leaking head gasket but it showed that it was fine, my z on the other hand has no big signs of a head gasket leak ( except the way it's running) but showed the chemical turn blue to yellow rather quickly?
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Old 11-20-2013, 12:53 PM   #12
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Re: 94 z28 hesitation and rattle upon acceleration?

Also, the hesitation and rattle doesn't seem to start until the temp gauge reaches 180+ degrees, the rattle is definitely coming from the top end of the engine. Drivers side head is where the rattle seems to be coming from. Also, if I put in park and rev it doesn't bog down.

Last edited by dallasz2876; 11-20-2013 at 12:55 PM.
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Old 11-21-2013, 07:26 PM   #13
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Re: 94 z28 hesitation and rattle upon acceleration?

Did an oil change Saturday 11/16 using Mobil 1 10w-40 high mileage it was I need of one, last oil change I used Mobil 1 5-30 it seems the rattling has stopped thanks for all the advice
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Old 11-21-2013, 07:26 PM
 
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