Got into my 91 L98 today to drive to a meeting and it started right up as normal, but had a bad miss so I took my daily driver. When I got back home started it up again and same problem so I let it run to see if any changes as it got to normal temperature. No change to the miss/hesitation at temp. I last drove it last weekend and no issues. Acceleration since I bought it last year has always been strong and smooth. Haven't had time to hook scanner up to see if anything shows up and hope to do that tomorrow.

Any thoughts on what has caused such a drastic change from last Sunday when it ran great?? As I said, it starts right up as it always has, but now stumbles and almost stalls with throttle increase.

 

I looked for any codes today and it was clean. Then I recorded data from my 91 while engine off and again with it running. with my Actron. Problem now is I can't get the recorded information to upload to computer so I post it to see if any of you can point out any issues. A couple of readings that I'm wondering about are:

BLM CELL --- 204
EGR DIAG SW -- CLSD
ERG DUTY (%) -- 0
INJ PW (mS) --- 15.7
INTEGRATOR L -- 0
INTEGRATOR R -- 23
KNOCK RET (0) --12
LOOP STATUS -- CLSD
MAP ("HG) -- 17.88
MAP (V) -- 0.00
02S CRSCNTS L -- 0
02S CRSCNTS R -- 0
02S LEFT (mV) -- 0
02S Right (mV) -- 324
PORT VAC (V) -- 0.00
THROTTLE (%) -- 38

Anything in those attract attention??

 

I don’t spend a lot of time on this side of the Forum but first I’m assuming these numbers are at idle seeing the engine in closed loop...MAP voltage shouldn’t be 0 volts and when you show your O2 sensors since they switch from 200-800 Mv one number doesn’t tell the whole story...do the O2 sensors switch from 200-800 Mv at idle ??...I’d have to brush up on my Integrator and Block Learn....does your Actron show misfire counters ??....if not do you know how to do a cylinder drop test ??...connect a test light to battery negative and remove 1 plug wire at a time...let the spark jump to the test light and see if there is any cylinder where you don’t hear a drop in RPM...the cylinder where you don’t hear a drop is the “dead” cylinder which is not contributing !!

 

I'm also confused by the 02 sensor readings. The one thing that I'm kind of ruling out are a fuel injector problem since if I floor it (after the initial stumble) it accelerates very strong with no issue. At idle I do notice when in gear it bucks a bit but doesn't stall. Am I wrong to take an injector issue off the table??

 

What confuses you about the O2 readings ??...like I mentioned they should be graphed..,and that MAP voltage should not be 0 volts !!

 

My confusion is with 02S LEFT (mV) -- 0
02S Right (mV) -- 324

Should one show 0 while other shows 324 ? When looking at all the readings I'm wondering if the Actron being used is capable of properly reading everything. I fully admit to being a big dummy with what I'm looking at. With that being said, I have great appreciation for information you have provided, such as testing for a misfire.

 

Ignore O2S Left, you only have 1 oxygen sensor and it is represented as O2S Right on your Actron.

Jared

 

Only 1 O2 ???...thanks for clearing that up !!...when I was looking at the L98 wiring I only saw 1 O2 sensor but wasn't 100% sure...maybe I'll just stick with C5's so I don't embarrass myself...LOL !!!

 

This has been a fun post. Sometimes too much data gets in the way of solving the problem. I LIKE the data, don't get me wrong.

Your MAP is the sensor in question I believe. It's only purpose is to tell the ECM what volume of air is heading into the engine. Stumbling and other issues are common to MAF or MAP failures. My Buick DD had it's MAP sensor fail. While cruising pulling a trailer the car was fine, under loads is when she had trouble like on inclines. It started as an occasional miss and got worse.

On your data you gave us, the missing MAP voltage has me wondering. When my MAP sensor finally died and gave me an over voltage code for the MAP. I replaced it and the problems under loads went away, once again nice and smooth.

In "Closed Loop" mode the ECM is primarily concerned with three important sensors, MAF/MAP, O2 sensor and the Coolant Temperature Sensor(CTS). Generally I look at the O2's first then the MAF/MAP and finally the CTS. All three have a major effect on the operation of the vehicle.

When the MAF/MAP goes out the engine looses the ability to "KNOW" exactly how much air is being ingested by the engine, the result is bucking and major drive ability issues or not running at all.

The O2 sensor will start to have smaller weaker signals where the peaks top and bottom start getting narrower and then start to lag behind which makes the engine A/F mixture go nuts causing the mileage to go down and eventually it can damage your catalytic converter with too much carbon. Oxygen sensors were made to be replaced every 50 k miles or two years.

The CTS is the really fun one. If the CTS thinks it is 20* ambient then it will activate the extra fuel to simulate a choke on the engine. A bad CTS can also fail where it thinks the engine is already 200* before starting, this causes the engine to be very hard to start as it does not add extra fuel on a cold start up. CTS on your car is likely where mine is on my 1988 C4 under the throttle body. I like to check it out every few years to be sure it is still accurate as it is a critical component of your C4. The factory service manual has a chart that has the resistance values next to the ohm reading you are reading telling you what temperature the CTS "sees". The CTS has three wires going to it, one is a 5 Vdc reference voltage, One ground and one signal (return) wire. Keep these terminals clean as corrosion causes resistance and can change the output value going to the ECM.

I have a gut feeling that you are dealing with a MAP sensor on its way out of this life. What you are describing is not likely just one injector causing all of these issues. Injectors should be removed and cleaned every so often, if your are original then this is even more important. Removing them to get them cleaned professionally is a wise thing to do unless you would prefer to replace the entire set. Me I am a BIG believer in B&G 44K Fuel Injection System cleaner as it makes a huge difference whenever I would notice that my idle was not glass like. A can of 44K every couple thousand miles is a great way to have a good clean fuel system. A set of NOID lights helps to verify that each injector is getting power never hurts to have in your tool box

If I had to replace the MAF on my C4 I would also make a point of replacing the relays that control it's functions. I am not as familiar with your year and it's parts, I know they switched to a MAP instead of the MAF at some point during the production run of the C4's. The MAF on my car has two relays that work with it. Thank goodness for Rock Auto.....

Best regards,
Chris

 

Chris, ABSOLUTELY !!...those are the Data Pids I always look at whether a Corvette or a 4 banger Honda...O2’s, Fuel Trims, (or Integrator, Block Learn), ECT, MAF, IAT, MAP (BARO)...basically the engines “vital signs” as it were !!

 

Many, many thanks to lotsofspareparts, C5 Diag and ctmccloskey for the information you have provided. It gives me a direction to head in during the holiday weekend. I already replaced a bad CTS a while back because it wasn't sending the proper information temp info causing it to run way too rich and stall when warm. In this case it idles ok (cold and hot), but has a stumble, like a miss during acceleration. Once it makes it past that it runs strong all the way up the rpm range. As I previously said, it runs rough when in gear (drive and reverse) to the point of almost stalling.

If I ever figure out why the recorded data from the Actron won't up upload to my computer I will post all the data.

 

With that 0 voltage MAP I'd look at that first....it's a 3 wire sensor...a 5 volt ref., a signal wire, and a "ref. ground" (ground)...just unplug the connector and see if you have 5 volts on any of those 3 wire !!...good luck !!

 

what's your fuel pressure, key on engine off, then at idle?

 

You are very welcome, CorvetteRules!

Not too long ago I purchased a Snap On MT 2500 scanner from a fellow member of this Forum. I like having tools to make my life easier. This scanner is now available from a couple hundred on up. The one I got had the cartridges for my C4 as well as many brands of Asian vehicles. I also have a hand held scanner but the difference in the Snap On was incredible. The Snap On requires time to learn how to operate it properly but has more features available in the long run.

I want to go to a data acquisition system at some point so I will probably buy a software and cable anyway. The big limiter is the speed at which these devices communicated, it is a bit slow (sorry bad pun). Being able to see data in real time is a great feature to have.

Good Luck getting that Corvette of yours back on the roads where it belongs! It won't be long...

 

Well I have my evening reading assignment, open up the FSM and read up all I can about the MAP sensor.

 

One question it does bring up is shouldn't such an issue set a code? The first thing I did yesterday was pull any codes and there were not any.

 

One more question, I was looking last night at MAP Sensors in case I find that mine is indeed bad. I've had no problem finding a number of vendors that sell the sensor, but it seems there are a wide range of ratings on the quality of some of them. Even the AC Delco part is getting a bad rep because many report the Chinese supplier AC Delco uses is producing a horrible sensor.

Any input from those of you that have had luck with a specific supplier would be appreciated.

 

I believe the one I used was made by DELPHI and it has worked great!
For your 1991 L98 Corvette it is available at RockAuto for $40 plus shipping. I have had pretty good success with Delphi (so far). I found a AC Delco shop nearby that beats R/A on their prices and all they sell is Delco. Having 4 GM cars makes this place a good connection for decent parts.

R/A is a great resource but be aware they do ship bad/faulty/defective parts sometimes. The first Delphi MAP sensor I bought from them was used and put back into a new box. When it arrived I had somebody else's Faulty MAP sensor instead of the new one. R/A assumed that I was trying to pull something and would not replace it with a good unit. I shipped it back to never hear any more about it.

I ordered an exhaust system and four O2 for a car and one of the O2's did not work out of the box. Again, I had to eat it as R/A was not exactly happy to replace it after shipping it back to them. I had to pay for one locally which killed all the savings from R/A in the first place.

Be careful buying electronic or electrical parts as returning them is a bit harder. I wish there was a test you could perform to see if a O2 is any good before you install it.

ZIP products has your MAP sensor but for $48 plus shipping. I don't know what brand it is but it should work okay for you as an alternate to R/A.

Best regards,
Chris

 

BEFORE replacing the MAP sensor you should ALWAYS check the wiring !!...your 91 MAP is probably a 3 wire sensor.... a 5 volt reference, a low reference (ground), and a signal wire back to the PCM...I don't know the wire colors for the sensor but if someone can supply them you can check the 5 volt ref. and ground by unplugging the connector and with key ON read the voltage between the 5 volt ref. and ground with a multimeter...if you read 5 volts the 5 volt ref. and the ground are good....you killed 2 birds with one stone..with the connector plugged back in and back probing the connector it should not read either 0 or 5 volts with key ON at the signal wire...I believe my MAP sensor on my LS1 reads 4.85 volts with the key ON...if your MAP reads 0 volts on the signal wire plugged in then 5 volts when you unplug it your MAP sensor is most likely BAD !!

 

Thanks guys. I was tempted to order a MAP sensor on Monday just so I'd have it this weekend to pop in if testing did prove it to be bad. It's not big bucks, but after thinking about it didn't want to get into the routine of throwing parts at it without checking. I'll let you all know how the weekend turns out.