93 B2600i runs like crap - cant be computer

[EGib]

Hello: like many others I have an issue that I cannot figure out. My 93 B2600i 2x4 with 205k suddenly started sputtering, stalling, & general refusing to run. It will sometimes start & run ok until warmed up - drives fine for 2-10 min, then start to sputter again. opening the throttle fully will not increase rpm's, but pumping the throttle will increase rpms slightly - just enough to limp back to the shop..

At first I thought it was a computer/electrical component issue because it would run ok when cold then start breaking up. I have access to Alldata & have just about memorized it for this vehicle. I have a good parts guy who has let me 'test' some components at no cost. after testing the voltages at the computer per the Alldata pinout, everything showed good as per the pcm testing section. so I replaced the pcm with a rebuilt unit ( O'reily). No change. OK maybe bad unit; Ordered another - still sputtering. OK 3 diff computers...2 'new' rebuilds - same symptoms .. not computer then.

Voltage testing at the PCM showed ox sensor stuck on lean, so by adding fuel(brake cleaner) to the PCV hose I verified a lean condition - as long as you spray, it runs fine..... OK so lean - replaced fuel filter. no go. tested fuel pressure. at idle in specs at 38psi. remove fuel press vac hose - in spec at 41 psi. at idle when throttle opened & engine bogs down, fuel pressure increases to 41. No drop in pressure when bogging down tells me not a fuel volume issue . . fuel pump ok even though max fuel press test failed - spec 64-85 - I had 58.

OK.. Computer opens injectors & adds more/less fuel because of TPM, MAF, Ox sensor, & feedback from Coolant & air temp. Coolant sensor ok ohms for temps, Air temp sensor ohms in range. TPM tests ok at throttle body connector & ECM input ( same readings). OX sensor ok - reads rich when brake cleaner added - then leans out when stopped. MAF tests ok by voltage - same at sensor & at PCM per Alldata supplied test voltages. Cleaned MAF no change Replaced MAF with new - no change. Checked air intake hose between MAF & throttle body for cracks - nothing.

A major vacuum leak would cause this symptom, but no hissing of vacuum can be heard & I have used several cans of brake cleaner looking for a leak. Removed & plugged air baffle just before throttle body ( plastic box bolted to back side of valve cover) - no change. Also blocked off crankcase vent at air baffle & blocked the PCV hose in case that was leaking & leaning things out. no change.

Not a spark issue, but Just in case, installed new dist, Ignitor, and fuel press reg (old tested ok) at same time as MAF - no change. removed pulsator valve & fuel press reg, blew out fuel rail - reinstalled without pulsator valve, no change.

OK - I am lacking fuel in the cylinders, all sensors test good, fuel press ok, no wiring issues between sensors & computer (readings same both places), computer assumed ok ( 3 diff & no change). Hmmm Computer has 2 circuits to fire injectors. Injectors fire in pairs (1&3, 2&4) when warmed up, All 4 injectors fire at the same time when cold. I need more fuel... Injectors are not opening up enough to let the proper amount of fuel in - ground issue? cleaned / re-installed battery, body, engine grounds. Tested all grounds coming into computer & even used a jumper to provide ground to the computer housing & pins 1r, 2a-d, & 2p. no change

I can trick the computer into thinking it's cold & make it double fire the injectors to get more fuel into the cylinder. Radio shack for resistors. Air intake 3.5k ohms at 185f, 30-36k ohms at 75 f - so 60k should convince it that it's real cold. Coolant temp sensor 15k ohms is 0 f unplug & install resistors - start up & it idles better, but still bogs down on throttle opening?? Noid lights show injectors circuits firing ok - steady flashing - faster slower w/engine speed.

What did I forget? Just in case - the pinouts for a 92 b2600i computer is the same, but different part number. Order 3rd computer - no change! I'm running out of options here!

OK fuel pump acted ok at operating pressure, but failed max pressure test - put in new pump. Checked tank for excessive trash & water in fuel - ok. started up...still sputters & bogs down. It will idle cold but die if throttle is opened.

Maybe IAC stuck open some & leaning things out, even though it will go into the high idle, idle down sometimes. Banged on Idle air control to see if sticking. no change. removed & cleaned / soaked with throttle cleaner. Cleaned throttle body w/ TB cleaner. Replace IAC with used unit 'guaranteed good'. No change

OK Mazda Gurus: What did I forget to try? I'm thinking next run a jumper wire from the MAF directly to the computer. I keep hearing that these computers are bad & agree this acts like one, but 3 reman computers installed & have the same symptoms! Surely all 4 would not act the same if bad? Im baffled.

In reviewing this I realize I have not removed & blocked the vacuum line to the booster. I'll try that tomorrow.

[Tyranitar]

You can very well have 4 bad computers. The problem with most remans is they are computers that failed due to the leaking capacitors. The capacitors were replaced and the obvious corrosion was repaired, but the contaminated humiseal was not removed and replaced. This humiseal has the acids in it, which causes additional damage over time. These companies have put these rebuilt computers in storage, most of which have failed again due to additional corrosion. To my knowledge, www.avproecm.com is the only company that removed the contaminated humiseal and they do in fact guarantee their rebuilds for life.

I have heard people tak about injector harness problems. Never personally experienced it, but there was member of this forum who told me it happened all the time. I would check the injector harness and make sure it is in perfect condition.

Do not attempt to fool the computer, that will only make matter worse.

Your MAF could very well be part of the problem. I do not see how a simple voltage test could tell you it is working. If the voltage were out of spec if it is bad, but just cause its in range does not mean it is good. What does your MAF do as airflow through it changes? Have you tested the MAF with a scope?

[EGib]

I have no scope to test the MAF, but the voltage called for by alldaya is in range. At idle I have 2.6-2.7 volts & this increases smoothly up to 4+ volts at 4000+ rpm - dont like to scream it higher in free revs. The thing is, when it's cold & running fine & then after it warms up & starts running stupid (assuming it decides to run fine cold) the voltage & response is the same. Just on a whim, I ordered another MAF last night & installed it this am, smae readings as the old one & no changes in running stupidly.

I guess I'm gonna have to roll the dice on a $170 'puter send off like you suggest. Like Ellie said: Occam's razor appears to apply here - I've eliminated everything else..

[EGib]

Well - I'll eat my hat... Sent the computer off to AvproEcm - $171 & 3 days later, I installed the rebuilt computer & fired it right up!! So much for a A1Cardone reman computer - I tried 3 - The wanted $85 each & they were all bad out of the box!

Thanks Tyranitar I'll bet you like to hear that you were right. So much for mu trust in A1Cardone computers...