mgray10

iTrader: (1)

Bought a used 88 iroc-z automatic with a 96 350 vortec. The only thing that was retained from the donor vehicle was the block, heads and rotating assembly. The entire air/fuel assembly is the older L98 style setup that was originally on these cars (not sure if it was carried over from this car or not).

Here is the problem: The car will just all of a sudden start running very strange, wont hold the rpm, and even detonate sometimes. Needless to say, I need to fix this ASAP!

I am assuming this is "limp-home mode", if it is not, I sure feel like I am limping home. Worst feeling yet, especially when the ricers want to race!!

When it does go into this mode, the rpm change (which is usally from wherever i am at, to roughly 700 and then back to where i was at and then back down and back up and down...etc you get the idea) is the same each time. However, I have not observed any conditions which make it go into mode all the time, such as temp, barometric pressure, humidity, or type/origin of fuel (of which I always buy the premium octane rating available). Additionally, the time period from engine start, to mode beginning is never the same, and some days it wont even do it.

I have looked at the MAF sensor, and the elements are still connected; nothing appears out of the norm. The wires on the connector and leading back to the relays all look unsevered and solidly connected as well. <- it certainly acts as though it is a loose wire, because of the intermitency of it.

I have not replaced any parts, as there are many I can think of that could contribute to it mucking up like this: 02 & MAF sensors, power and burn-off relays, IAC valve assembly, TPS, IAS, ECM and a few other various possibilities. To my knowledge, the only aftermarket mods to this engine the owner mentioned and i have confirmed where ignition coil, manifold base (to adapt TPI to Vortec heads), distributor cap, headers and cam.

I did a simple ground-diagnositc jumper check and cam up with codes 33 (MAF voltage too high) and 34(MAF voltage too low). Other than systematically replacing each part until it stops, is there any other method to approaching this?

Sorry for the long post, but I certainly do not like it when other people say, " my chevy is running wrong, whats wrong with it???", in all caps!

Thanks guys
Mike :lala:

sqzbox

Might want to check the fuel pressure and see if the pump is working right. Incorrect fuel pressure = lean air fuel = spark knock= comp. controled spark retard up to 20*. In other words, it runs like s**t. Sometimes good, sometimes bad. What happens when you give it WOT when it messes up? and have you ever had problems with it staying running after starting and warm up unless you mess with the gas pedal? Also, I found two wires arcing on my external coil once, The finger probe works very well It caused it to surge and miss.

mgray10

iTrader: (1)

Forgot to mention, the exhaust is very rich! I took it in the have the tire plugged (damn kids playing with glass on the street, did not see it at night, with my silver stars, the roads all look like they have glass in 'em!) and the guy said hey man is this yours and i said yeah. he says get this damn thing out of here its stinking my shop up, other than that I your tire is ready to go!

Sometimes WOT will clear it up for 10-15 seconds or less, then it comes right back. Once it starts doing it, it does it for the entire day. But then somedays when i let it sit over night it wont do it at all the next day!

With the cam it has in it, it always sounds like it is going to stall at idle once it warms up, but never does. And when it goes into this mode, and starts to drop to the idle rpms, its scary becasue you think its going to stall but then it will come right back up for a few seconds to what ever rpm you have the pedal at. If you try and give it gas while it at a low rpm (initiated by my problem, not because i have the pedal at a low rpm) spot, it will pause and continue its duration of low rpm (this is probably around 1 second) just as though you kept it at the rpm you were at before it started dropping.

That is one of the inital things I had to fix, the work this guy has done is shotty at best! He had stuffed the coil back behind the manifold base, sandwiching it next to the firewall. With the vortec heads, you have to fab your own bracket for the external coil so he said **** it i guess and just rigged it. He even installed his own alarm and i ripped the POS out, exposing where he had used scotch tape to make splices and butt joints!!

There are a host of other issues i could ramble on about, boy do i feel like egging his house and proceeding with an **** whoopin! oh well, my fault right!

Mike

sqzbox

First, I would address the code 33 & 34 problem. Erronious readings by the MAF sensor will cause lean-rich problems and in turn, the comp. will try to compensate by adv. & retarding the timing and generaly screwing everything up! Running rich for an extended amout of time will also carbon up the oxygen sensor and cause more problems. It's like trying to smell something when you have a cold! Perhaps your cam and intake system is not compatible with your prom chip.

mgray10

iTrader: (1)

Well, if the ECM registers codes 33 and 34 by direct measurement of the MAF sensor voltage, it would indicate a faulty MAF sensor, relay, wiring, or voltage, no?

I will pull the o2 sensor and check that out, however, even if I found that it was garbage and replaced it, that still would not make the ECM say that the MAF voltage is too high or low, would it?

I am in the process of organizing a chip burning system, as I am sure that they have either left the stock chip in, or purchased a shelf chip like the JET, etc.

Do these computers think in the abstract? For instance, could it register strange oxygen levels from the O2 sensor, and then store a code for the MAF sensor being faulty? It is a semi-logical assumption, however, the computer is assuming the the O2 sensor is functional and not fouled. <- I can't imagine someone designing a system like that and thinking it is a good idea...

Mike

sqzbox

Gm's come a long way since 1988. It all boils down to A/F ratio.
Bad MAF= incorrect A/F = 02 sensor smelling something wrong.
Then it's called Domino's! Everthing starts messing up, MPG, Emissions, and Performance. Remember, the trouble code may not point to the problem sensor, but the sensor may set a code as a result of another problem.

mgray10

iTrader: (1)

okay instead of keep replying and wasting board space, i PM'd you.

Anybody have ideas about where to find a REAL mechanic, not just any old ASE cert fellow? "speedshop" type person, someone who will understand why someone would put a vortec engine in a car and why my exhaust is choppy as hell, someone who wont think its a whole in the muffler and automatically try and sell me a new one! obviously, looking up mechanic in the phone book wont work. hmm, interesting!