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97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

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97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Old 01-21-2019, 09:25 PM
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97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Having an issue with my 97 z28, recently I had to install a new opti, was extra careful to make sure it was installed correctly, replaced crank position sensor, new plugs and wires, no thermostat atm. after finishing it up, it runs and drives great, no misfiring or anything, but when I'm driving or parked I can ease into the throttle it'll accelerate/rev up just fine, but if I floor it, it'll start bogging out, backfiring and pretty much falls flat on it's face. any suggestions or advice on why it's doing this?
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Old 01-21-2019, 09:44 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Is it possible the rubber intake elbow is torn, or that the elbow is not fully over the throttle body, particularly at the bottom?

Without a t’stat it's possible the coolant isn’t getting hot enough to allow the PCM to enter closed loop A/F management? Is it cold where you live?

SES light on, any codes?
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Old 01-21-2019, 09:56 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Injuneer View Post
Is it possible the rubber intake elbow is torn, or that the elbow is not fully over the throttle body, particularly at the bottom?

Without a tístat it's possible the coolant isnít getting hot enough to allow the PCM to enter closed loop A/F management? Is it cold where you live?

SES light on, any codes?
it does have a couple small tears around where it sits over the tb, but it's been like that since before the issues started, it's in the 30's here now, and was in the 50's today, so that may be something to look into, I do have a t'stat, just haven't put in yet. there is an ses light, and I'm gonna put a scanner on it at work tomorrow. crank sensor was broken and I thought that would be the issue but after replacing it and resetting the computer via battery disconnect, it still does it. the opti came from oreilly, don't really know how good they are tho. did forget to mention the mods it has are a 52mm holly tb, 3" dual x pipe exhaust, and hooker long tube headers and k&n cold air intake.

Last edited by Zendega; 01-21-2019 at 09:58 PM. Reason: left something out
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Old 01-21-2019, 10:31 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Zendega View Post
Having an issue with my 97 z28, recently I had to install a new opti, was extra careful to make sure it was installed correctly, replaced crank position sensor, new plugs and wires, no thermostat atm. after finishing it up, it runs and drives great, no misfiring or anything, but when I'm driving or parked I can ease into the throttle it'll accelerate/rev up just fine, but if I floor it, it'll start bogging out, backfiring and pretty much falls flat on it's face. any suggestions or advice on why it's doing this?
There are a ton of possible causes here and it's just going to have to be process of elimination.

Put the old plug wires back on and recheck wire sequence, to eliminate those possible causes.
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Old 01-22-2019, 08:06 AM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

you have a check engine light on and a torn intake duct....find out WTF the light is for and replace the torn intake.

Beyond that (read pointless to determine what may be wrong until you deal with the two known problems) check fuel pressure using a gauge while doing a SUSTAINED WOT run.

After dealing with the two problems and fuel pressure checks out....the store brand Opti could be a likely suspect

If your PCM has not been tuned for the 52mm TB and your car is a auto....your tranny will soon have issues
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Old 01-22-2019, 02:52 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

The crank position sensor has nothing to do with how the engine runs. It is only there for OBD-2 misfire detection. The PCM gets all the info it needs from the optical cam position sensor in the Optispark distributor.

If the rubber elbow is cracking around the throttle body, there's a good chance it's rotted enough to have tears anywhere, particularly if you pulled it off and reinstalled it as part of the Opti replacement.

As noted above, pull the codes.
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Old 01-22-2019, 09:42 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Injuneer View Post
The crank position sensor has nothing to do with how the engine runs. It is only there for OBD-2 misfire detection. The PCM gets all the info it needs from the optical cam position sensor in the Optispark distributor.

If the rubber elbow is cracking around the throttle body, there's a good chance it's rotted enough to have tears anywhere, particularly if you pulled it off and reinstalled it as part of the Opti replacement.

As noted above, pull the codes.
ran a scanner on it today and the codes it was showing were crank sensor circuit performance, and maf sensor A circuit and maf sensor A circuit performance. so I'm guessing somehow it's fried the 3rd maf in a row. any ideas on why it keeps killing maf sensors? also didn't have this issue when the first two went out. when the first two went, it idled rough and drove rough.
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Old 01-22-2019, 10:54 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

What are the EXACT numerical codes? Your MAF descriptions sound like generic GM code description, not specific to the LT1 engine.

Air bypassing the MAF sensor can set a code for irrational MAF flow, when the PCM compares the MAF reading to the mass air flow calculated by the speed-density method. A torn, or incorrectly installed rubber intake elbow allows air to bypass the MAF sensor, common cause of “when wot falls on its face”. No guarantee that’s the problem, but worth CAREFULLY examining the condition of the elbow.

CKP code is important too..... there are two for the LT1, and one can be the fault of an Opti that is not correctly indexed to the cam dowel pin. Is it the same code you had BEFORE replacing the CKP sensor, or did it just pop up after you replaced the Opti?
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Old 01-22-2019, 11:09 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Injuneer View Post
What are the EXACT numerical codes? Your MAF descriptions sound like generic GM code description, not specific to the LT1 engine.

Air bypassing the MAF sensor can set a code for irrational MAF flow, when the PCM compares the MAF reading to the mass air flow calculated by the speed-density method. A torn, or incorrectly installed rubber intake elbow allows air to bypass the MAF sensor, common cause of ďwhen wot falls on its faceĒ. No guarantee thatís the problem, but worth CAREFULLY examining the condition of the elbow.

CKP code is important too..... there are two for the LT1, and one can be the fault of an Opti that is not correctly indexed to the cam dowel pin. Is it the same code you had BEFORE replacing the CKP sensor, or did it just pop up after you replaced the Opti?
the maf codes were p0100 and p0101. can't remember the crank sensor code, but it was just an issue with the connector. the maf on it is a used one from another z28 that had over 200k on it, was just a quick cheap fix because at the time I couldn't afford a new one. I've checked around the intake duct where I has the couple tears and at idle and giving it throttle, there's no signs of it pulling air in. I can double check to make sure, but I won't be surprised if it's a maf issue again.
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Old 01-23-2019, 11:33 AM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Getting confusing. You indicated when you checked most recently, you found a code for the CKP sensor, but when I asked about it, you indicated "it was just an issue with the connector." Do you still have a CKP code, or don't you? Which one was it? If you still have a CKP code, let's rule it out with a further check.

If you have access to a high end scanner at work (like a Tech-2), try and read "CKP:LoResolutionAngle". That is what sets P0335 and P0336. I have diagnosed a case where it indicated a -14-degree reading, indicating that the crank position measured by the CKP was off by 14-degrees from the (pseudo) crank position indicated by the low resolution cam position from the Opti. That was setting P0336 because it is outside the range of -10-deg to + 7-deg. Tracked it down to a cam dowel pin that was too short and ended up in the cloverleaf of the Opti drive, and not in the correct hole.

While P0335 is set by very large deviation of the CKP:LoResolutionAngle (outside the range -21-deg to +51-deg). it also sets when there is no signal at all from the CKP, which could have been fixed via the connector.

P0100 for the MAF sets when there is no change in the MAF output frequency over a period of 1 second. A common cause of this, particularly on the 1997 model for some reason, is a break somewhere in the wiring harness of the pink wire that supplies 12 volts to the MAF. Not an MAF problem, but a wiring problem peculiar to the 1997. And could also be a faulty MAF. Take plug off the sensor, check for 12 volts from the harness pink wire to ground. When codes sets, PCM defaults to speed-density to calculate mass air flow. In speed-density, the accuracy of the IAT and MAP sensors is critical.

P0101 is the MAF sensor rationality check. This is the one that sets when the PCM compares the MAF measured mass air flow to the mass air flow calculated using the speed-density method. A large deviation (e.g. - intake air bypassing the sensor) will set this code, as will a faulty MAF sensor.
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Old 01-23-2019, 12:47 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Injuneer View Post
Getting confusing. You indicated when you checked most recently, you found a code for the CKP sensor, but when I asked about it, you indicated "it was just an issue with the connector." Do you still have a CKP code, or don't you? Which one was it? If you still have a CKP code, let's rule it out with a further check.

If you have access to a high end scanner at work (like a Tech-2), try and read "CKP:LoResolutionAngle". That is what sets P0335 and P0336. I have diagnosed a case where it indicated a -14-degree reading, indicating that the crank position measured by the CKP was off by 14-degrees from the (pseudo) crank position indicated by the low resolution cam position from the Opti. That was setting P0336 because it is outside the range of -10-deg to + 7-deg. Tracked it down to a cam dowel pin that was too short and ended up in the cloverleaf of the Opti drive, and not in the correct hole.

While P0335 is set by very large deviation of the CKP:LoResolutionAngle (outside the range -21-deg to +51-deg). it also sets when there is no signal at all from the CKP, which could have been fixed via the connector.

P0100 for the MAF sets when there is no change in the MAF output frequency over a period of 1 second. A common cause of this, particularly on the 1997 model for some reason, is a break somewhere in the wiring harness of the pink wire that supplies 12 volts to the MAF. Not an MAF problem, but a wiring problem peculiar to the 1997. And could also be a faulty MAF. Take plug off the sensor, check for 12 volts from the harness pink wire to ground. When codes sets, PCM defaults to speed-density to calculate mass air flow. In speed-density, the accuracy of the IAT and MAP sensors is critical.

P0101 is the MAF sensor rationality check. This is the one that sets when the PCM compares the MAF measured mass air flow to the mass air flow calculated using the speed-density method. A large deviation (e.g. - intake air bypassing the sensor) will set this code, as will a faulty MAF sensor.
ckp code is gone, just have p0100 and p0101 now, I checked maf and it was dirty, so I cleaned it up good, checked the wires to make sure nothing was broken, and still getting codes and the falling on it's face under wot. it did get a little better after getting rid of ckp code but not much
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Old 01-23-2019, 05:28 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

The CKP code does not affect the way the engine runs. The sensor is only there for misfire detection. 93-95 LT1’s run fine without a CKP sensor.

Did you check for 12 volts in the pink wire in the MAF harness connector? The breaks in that wire, peculiar to the 1997, are deep in the harness, not visible. Is the IAT sensor plugged into the rubber inlet elbow?
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Old 01-28-2019, 06:15 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Originally Posted by Injuneer View Post
The CKP code does not affect the way the engine runs. The sensor is only there for misfire detection. 93-95 LT1ís run fine without a CKP sensor.

Did you check for 12 volts in the pink wire in the MAF harness connector? The breaks in that wire, peculiar to the 1997, are deep in the harness, not visible. Is the IAT sensor plugged into the rubber inlet elbow?
Had a diagnostic computer put on it today, from what it's showing, my throttle position sensor is bad. at idle it shows it at 0.0, put it to the floor and it only goes up to 49% max and it won't run above 3000 rpms
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Old 01-28-2019, 11:24 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

At closed throttle, was the reading 0.0 volts, or 0.0%. A scanner shows both the voltage and the throttle position %, which it derives from the voltage. Similarly, was the WOT reading 4.9 volts, or 49%?

First, you want to read the closed throttle voltage. That should be in the range of 0.25 volts to 0.90 volts. A reading outside that range will set a code. Slowly rotate the throttle open. The voltage should increase smoothly and uniformly with no spikes or dropouts. WOT voltage should read about 4 volts higher than the closed throttle voltage 4.0 to 4.8 volts).

Did you visually verify that the throttle was fully open when you had the pedal on the floor? Are you sure the floor mat wasnít under the pedal?

If the engine can't accelerate beyond 3,000 RPM, it is not likely due to the TPS sensor. Either itís not getting air (plugged inlet or throttle blades not opening), or it's running out of fuel. A faulty TPS could affect the transition from part throttle to WOT. Your original post seemed to indicate the engine had no trouble reaching higher RPM, when you eased into the throttle. Now you are indicating it canít go past 3,000 RPM.
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Old 02-08-2019, 08:12 PM
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Re: 97 Z28 idles and drives perfectly fine, but when wot it falls on it's face

Thanks for all of the advice and suggestions guys, the wot falling on it's face issue was a clogged fuel filter, changed it and it's running good again, it was a new filter, but my fuel tank is ate up with rust so that looks like it'll be the next job for me.
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