So this is more of a case study than needing any advice, hopefully it may help somebody with a similar issue in the future. The car in question is a 1989 Iroc with a mostly stock L98. The only mods done to it (before I got it) are a cat delete and a 3" flowmaster exhaust system, and I converted it to a 1227730 SD system running SAUJP V7. The symptoms were that as soon as it would go into closed loop the BLM/Integrator would start dropping all the way into the low 100's (sometimes lower) and at the same time the wideband in the tail pipe would in open loop show around 13:1 and as soon as it would go closed loop it would head to an indicated 19:1 AFR, now this seemed counter intuitive to me and caused alot of confusion because they seemed to be telling me opposite things. I checked fuel pressure which was right around 45 psi, I checked resistance of all the injectors, which were in the normal range, I even did a pressure drop test on the injectors (where you pulse each injector individually and look for the px drop) they all dropped an equal amount. I then even pulled each injector plug one at a time and watched for RPM drop (they were all pretty much even) So I honestly didn't suspect the injectors. One strange thing I did find was that using an infrared thermometer the exhaust port for #2 cylinder was half what all the others were showing. This obviously indicated a misfire or a dead cylinder so I thought it must be ignition related, so I put a spark checker on that cylinder and it seemed to be firing perfectly fine, but still I figured it had to be something ignition related so in went new wires, cleaned the contacts on the cap and rotor, changed the module and put in a known good coil, The result? nothing exactly the same as before, low BLM/INT, rough idle, and 19:1 on the wideband. So after all that verification and knowing that the rochester multec injectors are notorious for failure I bit the bullet and threw in a set of injectors, I started it up today and the result...... Fixed! the BLM/INT are in the 128-130 ish range and the wideband is close to what I would expect. I've attached a before and after log (in excel format) in case anyone wants to see what a bad injector (at least one) might look like in a log on a SD TPI engine.

 

When I got my car 15 years ago that was the first thing I actually had to fix. Was running ok except at a light throttle steady cruise, then a bit ragged running, leaning out. Read up on those Multecs and, sure enough, new injectors fixed it.

 

Glad you found the problem

I remember asking you "Do you still have OEM injectors?" in your previous thread. I never got an answer (https://www.thirdgen.org/forums/diy-...go-closed.html)

Yep, the Multec, when they fail, they can mess up things. Good info on that on TGO. 11 years ago, mine had different symptoms, but engine was sputtering wet carbon residue through the tail pipes (pig rich). The PO had installed the O2 sensor on the primary tube on cylinder#1!!!!!! The ECM was not monitoring the 7 remaining cylinders. The O2 sensor was relocated not too long after to fix this mishap and converted to heated.

Could have been #1 getting lean and adding fuel which made the 7 remaining going rich. Or who knows? Results was the same as yours, overly rich.

Just like yours, injectors resistance was in normal range. However, after watching a YouTube video from Fuel Injector Connection, I rechecked resistance immediately after rapidly activating each injectors on/off with 12 volt probe. Resistance was dropping on some injectors once they were getting hot. I emailed FIC with my findings. He sent me a remanufactured set of Bosh 3. After installing, car ran like a charm.

My '89 IROC-Z low milage still has the Multec. They work perfectly. I am waiting for it though! I have a replacement set in stock. Might do it sooner than I think.

Peace

 

Interesting video, I will put my old injectors on the flow machine next week just for my own curiosity, but im sure the #2 is either completely plugged or at least mostly. I'm convinced now where those multecs are concerned it isn't a matter of if but when they will fail.

 

Post a video of it!!!!!

 

Hi. I am about to install a 383 with a FIRST intake in an 89 IROC and am getting conflicting numbers as far as what injector size to run , and really even what brand is any good-many choices. I have found 36-42 lb./hr. numbers , need some solid advice. am I to understand that you do sell injectors? I see that you have what may be a similar combo. Street car , may not ever see a drag strip , stick with air. Thanks...Dave

 

If I may, you should start another thread or research TGO or other forums. Plenty of formula and advice. You will get thousand different opinions. Here's a good one for ya:

https://www.thirdgen.org/forums/engi...st-intake.html

Hope this helps