Z28 won't run up

[Morley]

If the MAF was already replaced I would go to the relays first INCLUDING the fuel pump relay (yes it can cause code 33's). Here is a bit of troubleshooting you can do for the code 33/34

There are three codes that pertain to the MAF sensor: Codes 33, 34, and 36. Code 33 (mass air signal voltage high) is set when the computer sees a high mass air reading at low engine speeds. In other words, there appears to be more airflow through the MAF sensor than the engine can physically breathe at a specific throttle and rpm.

Code 34 (mass air low) sets when the computer registers less than 5 grams of airflow at idle or 2.5 grams/second of air when the engine is first started. The mass air wire inside the MAF sensor should heat up after the car shuts off to clean any contaminants from the wire. When this fails to happen, the computer shows a Code 36 (mass air burn-off malfunction). We'll address and troubleshoot each code individually.


The mass air sensor is a simple design. It's comprised of a wire that heats up when the car is running. The air entering the engine through the MAF sensor cools the wire. The more air ingested by the engine, the more air must flow through the sensor. More air means a cooler wire. The computer knows how much air is entering the engine by the temperature of the wire inside the MAF sensor. The air/fuel mixture is dependent upon having accurate readings from the MAF sensor. If there were a problem, a Code 33 or 34 would be set. Once a Code 33 or 34 appears, the MAF sensor will shut down and the car will go into "basic strategy" mode, aka limp-home mode, until the key is turned off for at least 15 seconds. You'll know the MAF has stopped working when, in Chris' words, "You let off the throttle and it feels like an anchor just dropped."

A Code 33 can be set for several reasons. It appears if the dark green wire (circuit 998) has an open circuit or a bad sensor; if the black, burn-off control circuit wire (circuit 900) has power with the car running; or if the system has a bad ground. You must check for an open circuit on 998 at the ECM on connector B12. There are several places to check for a grounding problem. The cast-iron-headed '85s and '86s have the grounds on the rear of the driver-side head. You'll have to pull the wiper motor to check. The aluminum-headed '86s and '87s have a pack of grounds next to the oil-temp sensor and filter. On the '88 and '89 Corvettes, the ECM, dash, and sensor grounds are on a lower lefthand-side bellhousing stud.


A Code 34 could be set for several reasons as well. A bad ground, an opening in the ducting between the MAF and the throttle body, incorrect minimum idle-air setting of the throttle body, or a worn throttle body can all cause a Code 34. Typically, a worn throttle body or a bad ground will set a Code 34 intermittently (every third or fourth time). To see if the minimum idle air is causing the problem, open the throttle slightly as you try to start the car again. If it's easier to start, check the minimum idle air.

To check the throttle body, simply depress the cruise-control bellows and observe if the throttle-body linkage moves laterally instead of only rotating on the shaft. If there is noticeable movement, your throttle body is worn and the engine is getting unregistered air through the gaps.

'86-'89 Corvettes use a powered fuse link and connector that supplies power to the fuel pump and MAF relays behind the battery. The '85 Corvettes do not use relays for the MAF, but instead use a module next to the ECM. This module rarely goes bad; but to check the module on '85 Corvettes, look for 12 volts at connection E on the MAF with the engine running. Then, run the engine for at least 5 minutes, turn off the key, and check for 12 volts at connection D (MAF burn-off). The powered fuse link is a common cause of intermittent MAF codes. Check the connection and wire for a possible open circuit.
The oil-pressure switch is another possible cause of intermittent MAF codes. If the engine feels as if it has an intermittent hiccup with a fuel pump and MAF code set, the oil-pressure sensor would be the place to start. If the switch is bad, the engine tries to start because power to the pump at startup is fed through the ECM's fuel-pump-relay drive (connection A1).

The relays changed in late '87. Relay No. 14089936 operated the MAF power, burn-off, and fuel pump. Although not always accurate, the connectors for the 9936 relay are the gray connector for the burn-off circuit and the black connector for the MAF power circuit. You'll notice that the new connectors are weather pack connectors to seal out moisture.
To check that the MAF burn-off is getting power, look for 12 volts at the MAF connector position E.
If you get a Code 36, MAF burn-off malfunction, it should immediately reveal itself at startup and it won't affect performance. '85 Corvettes will never show a Code 36, only '86-'89 Corvettes. You can check to see if the ECM is calling for an MAF burn-off by running the car for at least 5 minutes, and checking the black wire at connection F (circuit 900) on the gray connector. It should be grounded for approximately 30 seconds. This is the ECM grounding the wire to actuate the relay. If there is no ground, the ECM is at fault. If you have ground, check the power to the relay. If the relay is sending power to the MAF, check that the dark blue wire (position D) on the MAF sensor connector has power.


(See Previous Caption)
Chris has spent some time adding wires, meters, test lights, and jumpers, chasing down several specific reasons why Codes 34 and 33 will appear. Here is what he found: The dark green wire from the ECM to the MAF (circuit 998) can short to ground or short together with circuit 450 (black or black/white) and cause a Code 34. To get a Code 33, circuits 998 or 450 could have an open circuit. Recently, Chris has seen a few cars for which the only cure was to run new wires for both circuits. This cured the intermittent codes immediately.
Checking The Minimum Idle Air: The first step in checking the minimum idle air is to put the car into Field Service Test Mode. Do this by putting a spade connector into the A and B positions of the ALDL connector. With the key on and the engine off, the fan will turn on and the SES light will flash. Also, the Idle Air Controller (IAC) will fully close.

[89iroctpi5.7]

thank you found and replaced the codes cleared, it starts right up now but idle is too high 1200 to 1500 and 650 to 750 in gear what GIVES NO CODES REGERISTING EITHER anyways changed oil today also trying to calm down what i hope is just a noisy lifter (tap tap tap tap) comes and goes from right hand rocker cover. Ill be changing plugs wires cap button filters coolant sensor tps and tranny fluid tommorow. the miles are right around 100000 original and the wires and everything are orig not a well cared for iroc but Im trying to make up for it. im use to working on trucks this car is being quite the learning expierience! Appreciate all yalls input. I currently own a 1966 c10 sb stepside with 350hp sb and muncie combo just finished restore, a 1988 jeep grand wagoneer,(wife driving daily)92 s10(my comuter( and this Iroc purchased monday to fix up for her to drive daily instead of the 8 mpg wagoneer.

[wrightz28]

Originally Posted by 89iroctpi5.7

(tap tap tap tap) comes and goes from right hand rocker cover..

A couple of other possibilites if I may:

1-cracked/leaking exhaust manifold

2-Bad A.I.R. check valves.

3-the Electronic AIR diverter system is surprising loud sometimes.

[malletslinger]

Could also be a small carbon deposit or a sticky valve...how did the old oil look when you removed it? Was it really thick or have gunk in it or did it look normal?

[89iroctpi5.7]

aaaarrrgggg!!!! would not be so bad if it were one of above mentioned, bad news though CHANGED plugs wires cap coolant snsor valve cover gaskets today and now it wont start at all it backfires bad and the check engine light iss not coming on when you turn key on. the plugs were a b...h to change and the right valve cover gasket.... oh god....should have left alone the left was only one leaking had to move all the air pump stuff disconect wiring harness to all sensors and the starter to get slack out of the budle and get it out of the way and move the ing coil pack, but wnted to see if ther was anything obvious under there. You guys have been real supportive so any input on why it wont start now would be great.(did I fry the computer some how) YEAH THE OIL WAS GUNKY thanks again from the man with a dream of having a nice iroc z out here in Atlanta ....went back to garage foumd 30amp fuse blown next to battery check engine lights back ill try to find what shorted tommorow

[wrightz28]

30A? Mine is 20.

Double/triple check all the sensors for a snug fit, especially the MAF. Also make sure the wires are in the right firing order.

What are the plugs gapped at?

[89iroctpi5.7]

yeah I had started my wires on the wrong post must of been tired after the 3 hour valve cover ordeal changed tranny fluid and went for road test shifts okay drags out of second alittle. getting codes 24 and 33 now but no 36.(replaced burn relay) so I guess ill give and by a new maf and maybe I wont need starter fluid everytime I crank it.Could this speed sensor(code 24) be affected by the fact my speedo cable is broke right at the output of the tranny? Oh yeah pretty sure this tap tap tap is an ex man leak now.If any of you guys every need help with the old jeep grand wagoneers Im good with them (3000 pg shop manual)and one last thing the coolant temp sensor is the one right under the t body cuse I changed it and now my temp gage dont work these smilely guys say it all thanks again

[1BADRS]

3g's for a car that has been sitting for a year and only goes up to 20 mph? thats WAY overboard. you can get him down much lower than that. have you checked the linkage on the throttle control? may have come lose or someone may have tinkered with it. my dad tried to do that to my car without telling me to slow me down, i figured it out soon enough.

good luck

[Morley]

Originally Posted by 89iroctpi5.7

yeah I had started my wires on the wrong post must of been tired after the 3 hour valve cover ordeal changed tranny fluid and went for road test shifts okay drags out of second alittle. getting codes 24 and 33 now but no 36.(replaced burn relay) so I guess ill give and by a new maf and maybe I wont need starter fluid everytime I crank it.Could this speed sensor(code 24) be affected by the fact my speedo cable is broke right at the output of the tranny? Oh yeah pretty sure this tap tap tap is an ex man leak now.If any of you guys every need help with the old jeep grand wagoneers Im good with them (3000 pg shop manual)and one last thing the coolant temp sensor is the one right under the t body cuse I changed it and now my temp gage dont work these smilely guys say it all thanks again

Replace the MAF power relay and fuel pump relay if you didn't do them when you did the burnoff relay.
Yes your VSS code is because the speedo cable is broken. The VSS buffer is behind the speedo and reads the revolutions of the speedo head, you need it fixed.
The coolant temp sensor you changed has nothing to do with the gauge not working, it feeds temp info to the ECM only.
For the gauge, look on the driver side head between #5 & #3 cylinders, there is a single wire connector that hooks to the sender for the gauge.

I had a whole reply typed last night but the site was screwing up and lost it.

If you oil was black crude oil when you changed it, drain out 1 qt and ass a qt of auto trans fluid to the oil. Run/drive the car for 30 minutes and drain the oil (while hot) and change the filter (let the oil drain for an hour). Then refill with oil and new filter. The detergents in the ATF will break down any sludge deposites in the engine and liquify them so they'll drain out.

Transmission, you mentioned changing the fluid. If the fluid has never been changed and it is still red and doesn't smell burned, LEAVE IT ALONE! If you change the fluid and filter now you'll have trans troubles a little later down the line.

[89iroctpi5.7]

yeah yeah 2700 was alittle high, but these irocs with the 350 are fairly rare ,Ive been lookin for a couple months and most around here wnted 2000 more for theirs with more milage. the interior is mint needing only minor attention the body has no dents and just some nose dings and rear bumper scratches. two days and alot of elbow grease and its getting there. I still have to use starter fluid to crank it, but it roars down the road just fine. Alittle more cleanin and tweakin and ill be ready to hand it over to kelli. THANKS AGAIN FOR EVERYONES INPUT(i was having probs findin that temp sending unit)

[97cavalier]

i do not know what i would do with out this site.

[malletslinger]

Have you changed the fuel pump relay, or the fuel filter yet? If not you should.
Do you hear the fuel pump priming the engine when you turn the car to on or not? I think your starting problem may have something to do with the priming.