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Topic: GM EFI (Electronic Fuel Injection) Troubleshooting (Read 181 times) previous topic - next topic
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GM EFI (Electronic Fuel Injection) Troubleshooting
As per my usual, I searched around LDOF and the old Yahoo forum to see if there was a relevant topic in which to update, but I didn't find anything specific to the information I've posted below, so here's a new thread for future reference in case anyone has to deal with similar technical issues.

Background: I have a 22' MP (multiplan - think std config for the Dodge sportsman) built on the '93 Chevy (GM) G30 chassis with a 7.4L (454 ci) big block. It has both the throttle body injector (TBI) - which is sort of a glorified electronic carb - and an ODB1 ECM that controls  both the EFI and the 4L80E transmission (3+OD).

A few months ago, the tranny started acting up, and as I traced out the problem, it turned out it wasn't mechanical, but rather related to the ECM. So, for $200 (less $100 for the old board), I swapped out the OEM ECM for a remanned ECM from Cardone.

1993 CHEVROLET G30 7.4L V8 Electrical Parts | RockAuto(ecm+computer),2896

The transmission problem was solved, but a new fuel issue popped up. The ODB was throwing a diagnostic trouble code (DTC) 45, which is a rich fuel mixture detected. That was happening between open loop & initial start/idle, to right when the temp sensor (which sends a micro-voltage increase to ECM) told the ECM to go to closed loop. It would then sputter/choke for around 15 seconds, at which point it would resolve itself and the CEL went off.

Since I have the original GM shop manuals, it has extremely detailed troubleshooting charts and procedures. I was able to go through each step for DTC 45 until the final conclusion: the ECM. However, since I had just replaced the ECM, the possibility existed that it could actually be the plug & play PROM (programmable read-only memory). During the 90s & 00s, there was a lot of after market action where enthusiasts (especially with the SS 454 short bed trucks) were changing chips, heads & cams to boost stock HP from around 230 up to 375 and higher.

Anyway, as these GM vehicles have become older, there aren't any standard ECM replacement parts being made and/or supported any more. That's why there's a ready market for old PROM chips and re-manned ECM boards, but mostly on eBay. However, what is interesting, and the reason for this post, is there is a vibrant market for those who want to upgrade actual 70s & 80s carb systems for newer, modern EFIs.

Even better, the companies that are providing these after market systems are familiar names that stand behind their product lines. Rather than post everything here, I'm simply going to reference some an old enthusiast articles which have vendor listings at the end. While the market for performance upgrades may have tapered off for older SBC and big blocks,  a whole new market, which I guess I find myself in, is for those seeking to merely replace their old OEM ECM system.

Right now, we're running a diagnostic on the chip. If it turns out the chip is bad, but the ECM board is ok, then I need to find a chip for the right year/size. I've found a few vendors that have old ones for around $150, but what I'm really thinking about is simply moving over to one of the newer aftermarket ECMs. Besides the new computer, they have complete harness kits that go to the all the sensors. (They also have kits which provide sensors for those upgrading from manual carbs.)

I'll update this post later to indicate the direction I took, but I figure there are plenty of people out there with 80s & 90s Chevy chassis before LD switched over to Ford only in 96(?) that might end up looking for this type of info. Here are the articles I mentioned above that go through all the details and provide vendor lists

http://www.trucktrend.com/how-to/engine/0412st-chevy-truck-tbi-to-efi-conversion/

10 Common EFI Mistakes - Hot Rod Network

Here’s 10 Things to Help You When Converting to EFI
Re: GM EFI (Electronic Fuel Injection) Troubleshooting
Reply #1 –
OK, I just wanted to update the thread in case anyone else experiences similar types of problems and is looking for some troubleshooting guidelines.

It turns out the (replacement) ECM was at fault. My parts guy ordered another one from his warehouse, which he kindly extended to me without payment. He provides excellent customer service, and wanted to make sure we were able to isolate whether or not the previous re-manufactured ECM I got from him was defective. Once we swapped ECMs, I swung by and dropped off the first one I had purchased from him.

In parallel with these direct activities, I had a chance to exchange a few emails with a person over the weekend who was heavily involved in the TBI after-market performance business 10-15 years ago. I actually contacted him in preparation of perhaps needing to source a PROM chip in case the board was ok. However, without providing much detail, here's part of his reply where he got right at the heart of the matter:

 Those eproms rarely fail but the 16147060 computer used in the 1991-93 454 trucks GM did have a lot of issues.

So, guess which ECM I have? If you follow the RockAuto link to the anchor post, you'll see the remanned ECM I got is the one my expert mentions. Since I had this information at hand, I wasn't surprised when my mechanic told me it was the ECM I had replaced last month ie the new ECM worked ok.

To summarize, I swapped a re-manned Cordone ECM for another re-manned Cordone ECM. Same engineering, different board. The rig seems to run very well so far, so hopefully this finally fixes the problem. If issues continue to pop up, I may consider junking the entire OEM ECM in favor of a Holly or MSD after-market kit.

Of course, this gets into the heart of the debate whether it's worth it. At this point, the RV is still cheaper than anything new(er), by many factors.
Re: GM EFI (Electronic Fuel Injection) Troubleshooting
Reply #2 –
I've also pondered what to do with my TBI system. My 350 TBI is a good runner with excellent oil pressure so I expect it has a long life ahead of it (143,000 miles).

However, when it does need a rebuild, it would be really hard not to use some better performing parts by swapping in Vortec heads and something like an LT1 or LT4 production cam, swap to roller camshaft (block should have the provisions just not the parts), maybe headers.

Problem is, 1st gen GM fuel injection hates modifications, and tuning anything to run worth a damn is incredibly difficult. It seems to get their FI systems going in the 1980's, they did as much tuning (detuning) of the engines in question to get them to run well as they did dialing in the FI program. Lots of tiny ports and swirl heads and restrictive exhausts to get good burn and a nice hot signal to the O2 sensor. The 1st gen FI systems can only learn in a very rudimentary way and if I recall, can only learn on fuel, and then only in closed loop part throttle operation. Everything else the computer goes back to the basic 2 axis throttle position/RPM fueling table. Back in the early 00's, I built a 350 TPI firebird that I was very happy with, but the computer could never learn Colorado altitudes at wide-open-throttle and always ran a bit rich there.

So, if I find myself rebuilding the engine, I'm probably looking at going the aftermarket route for fuel injection, which is $$$. I guess the other option would be junkyard hunting a vortec FI system, but I'm not enthused about splicing another factory harness in vs the relative simplicity of buying an aftermarket harness. 
1989 MP
Re: GM EFI (Electronic Fuel Injection) Troubleshooting
Reply #3 –
Honestly, I would junk the old EFI and spend the $ on a new integrated system. If you check the links I posted above, or just google for some others, there are 3-4 major vendors providing complete, turn-key EFI systems. There are different packages depending on whether you want ECM, sensors, harness and/or performance parts (heads, intake man, cams, etc).

It's only after you've become immersed in the details that you come to realize that it makes total sense that there's a huge aftermarket for EFI replacements/upgrades. I've heard that somewhere in the neighborhood of 100m SBC 350 were made, both in the US and under license in different countries.

Assuming a significant number were old carb or rudimentary EFI, then it's easy to see why there are so many upgrade performance packages. With both the short & long blocks and various parts in large abundance*, with a little modern tech, you can create a pretty nice 'mighty mouse' engine that roars again.

* You could literally score any number of parts in outer Mongolia if you had to.