jaymitchell01

Good Day All - Weird problem with my 1991 Z, the car is rev limiting at 3000 RPM. This occurs while stopped or in gear driving down the road.

I've got no SES light, engine start and idles without issue, and car revs to 3000 without issue. I'm unaware of any other issues the car has. I've combed the forum and cannot find any problem/solution that matches what I have going on here, so here I am.

FYI - secondaries are eliminated, and has headers (no cats) and an aftermarket chip tune. Injectors we're recently checked (a couple months ago), and all checked normal. Have no misfires either. Only thing I haven't checked is my fuel pressure under load, but I replaced the entire fuel pump unit just 1 year ago, and hardly drove the car since then, so having trouble believing its the pumps, but maybe the gas sitting over the last 6 months caused an issue with the pumps.

My other suspicion is the diagnostic vacuum sensor under the ECM. Why the vacuum sensor? Because when I removed the secondaries many years ago, I had a similar issue, and realized I had to keep my secondary vacuum pump connected to the sensor. The vacuum pump doesn't seem to be working, so wondering if its that. Does anyone know how much vacuum that pump pulls? I'm planning to diagnose that today or tomorrow, would help to know.

Other question related to the diagnostic vacuum sensor: in the sticky for eliminating the secondaries, it says the sensor can be open to the atmosphere. Does that hold true regardless of the chip tune, or does the chip tune have to account for that sensor being open?

Thanks in advance for your feedback, much appreciated.

Jason
91 ZR1
97 Viper GTS

Dominic Sorresso

Jason,

The chip can be programmed to eliminate any need for the secondary vacuum pump. The vacuum pump is there to augment vacuum when u go WOT. In normal cruise, the motor produces enough vacuum to operate the secondary throttles. However, at WOT, MAP goes to ~ 100% of atmosphere, and so the secondary pump picks up the slack. The secondary MAP sensor is there to tell the ECM whether u have sufficient vacuum to operate the secondary throttles before turning on the secondary injectors. It's not a diagnostic sensor.
Have u checked ur secondary fuel pump? If the car sat around, may want to also look at fuel filter behind passenger side front wheel well.

Hib Halverson

Did the engine just start this problem or has it been there since you deleted the secondary port throttles?

Also, the aux. vaccum pump is irrlevant because you have eliminated the seconedary port throttles.

Has the calibration been changed to a chip designed to run with no port thorottles?

ZR-71

Not sure if the answer is there but this thread has some high idle discussion

https://www.corvetteforum.com/forums...-2000-why.html

jaymitchell01

Thanks for the replies thus far...

Dominic - Secondary MAP sensor? Where is that? The chip was programmed for the elimination of the secondaries (this was around 2004, so memory is a little fuzzy), and when I took everything out, I had this same problem, and what I ended up having to do was leave the secondary vacuum pump in the car and hooked up directly to the diagnostic vacuum sensor (see below, hopefully it pops up or provides the link). Been running it that way ever since...

http://i287.photobucket.com/albums/l...cuumsensor.jpg

Don't ask me why it's set up this way, the tune was from 2004 and I owed the guy feedback for a tweaked tune but never did because she ran so nice I didn't bother. To diagnose, I tried using a handheld vacuum pump on the sensor to see if the problem went away, it didn't. So now wondering if the handheld provided enough vacuum or not, that's why I asked if anyone knew how much vacuum that pump pulls. I may need to check it again with more vacuum (have an A/C vacuum pump I can try). If the handheld is enough vacuum, I'll turn to the fuel pump/filter.

Hib - the problem just started, and yes, the chip has been tuned for no secondaries. Been this way for 12 years. Just not sure if the above mentioned vacuum sensor is the issue.

ZR71 - Funny you should post that. Right before my rev limiting problem creeped up, I was troubleshooting a high idle that came up, around 1500-2000 RPM. Checked all the standard stuff, MAP sensor and hose were fine, got to the TPS, unplugged and plugged it back in and idle was fixed. Then this popped up. Strange.

I may need to buzz my tuner. He might be able to help me narrow this down pretty quick, but pretty sure its vacuum or fuel. Just wondering if the rev limiting is a stock thing or something my tuner put in the car as a safety measure.

Dominic Sorresso

Jay,

Your Z is a 91, so look under the ECM and u should have a MAP sensor there w a hose attached. The hose is used to measure vacuum and provide the ECM w a voltage which indicates whether there is sufficient vacuum for the secondaries to operate. If the voltage is >.7v, then the ECM allows the secondary injectors to fire. The calibration can be done such that u don't need to have the pump connected at all for the secondaries to operate.
Again, this may not be the issue but...

mike100

I remember once I plugged in my chip to my chip adapter backwards and the car still ran in limp mode. it still revved to 5000 rpm on a rudimentary pre programmed fuel and spark curve on the primary injectors...maybe you could try that just to see if the tune you have was programmed with the wrong mask or somehow has bad info on the rev limit value etc.

JimZRyd

My 94 had a similar problem not long after I got it. Had just completed changing out the injectors, coils, secondary actuators, fuel pumps, plugs and wires. Took it out and it ran like a champ! Hit the rev limiter and scared the crap out of me. Thought I'd broken something. Drove home fine but took her out a couple days later and no matter what it would not rev past 3K. Checked what I could and couldn't find anything. Finally took the plenum back off, took everything new I put in out, and then reinstalled it all. Buttoned her up and she's ran great ever since. Never figured out what it was. Just assumed something came loose. Who knows. Best of luck finding your problem

Jim

gbrtng

I think the LT5 is rev limited at cold temperatures - Hook up a Tech 1 and look at the basic input parameters - coolant temps, oil pressure, etc. - there is probably a defective sensor or a loose connection.

jaymitchell01

Dominic - I had no idea that was the second MAP sensor, but makes sense I guess. I always thought it was the "diagnostic vacuum sensor." Thanks for the voltage spec, is the calibration done via the chip tune or something else?

Jim - never found your problem? Well hell, that's encouraging LOL!

gb - know which sensors won't trigger a SES light? Most do don't they?

tpepmeie

The 3000 rpm cutoff means the ECM is in 'converter protection mode'. There are three potential causes:

Code 53 Low voltage
Code 61 Vacuum too high to close PTs
Code 56 PT vacuum sensor failure

You should grab a service manual and troubleshoot those codes.

Todd

gbrtng

A defective coolant temperature sensor won't - if it is open circuit, the ECM sees a VERY cold engine. There might be other sensors with similar failure mechanisms that won't or can't trigger SES.

Todd nailed the 3000 RPM cutoff - he's most familiar with the ECM code.

jaymitchell01

Todd - You are money. This is exactly the kind of lead I was hoping for. I will dive into my service manual tonight.

Two questions regarding Code 61:
1) Are you sure vacuum would be too high? I would think it might mean too low.
2) What does "to close PTs" mean? I assume PT = Port Throttle.

Hib Halverson

I think "gbrtng" is on the right track. Something is fooling the ECM into thinking the engine is "cold."

I agree with Todd's suggestion on diagnosing for those fault codes.

Also, when you key up, does the MIL come on during bulb check?

tpepmeie

cold engine will prevent the secondary port throttles from openings. But it will not invoke the converter protection 3k rev limiter.

jaymitchell01

I found the problem - the TPS was still out of range.

ZR-71 was on the right track. I looked into the codes Todd mentioned, but didn't see any issues and the problem remained, and then the high idle came back all of a sudden. I loosened the TPS to adjust, got it within range and all is well now: idle is normal and she now revs all the way to red line. I suspect that the voltage was just barely out of range, but not enough to make the idle run high and make me notice until now. Thankfully the high idle returned, or who knows when I would have come back around to the TPS.

So if anyone gets a rev limiting issue around 3000k, but the idle seems ok, don't rule out the TPS position/voltage. It could still be off. Thanks for everyone's help.

And now that that's fixed, anyone want to buy a yellow 1991 ZR1 with 39k miles? Hood needs body work and fresh paint all around, but mechanically she is tip top, and comes with lots of original parts and extras. Why am I selling? Because I'm a family man now and just don't have time for that and 2 sports cars, among other things, so have to part with one. If interested let me know, I'll have an ad posted as soon as the weather allows me to get some good pics.

Jason
91 ZR1 (yellow) - FOR SALE
97 Viper GTS