1991 TBI Setup Need tech HELP!

I installed a 1991 GMC 3500 7.4 TBI system in a 1972 402 powered truck for a friend. it's a little rich at idle (open loop) figured it would be. but it lacks power and the 91 donor truck had a knock sensor in the block but no wire on the factory harness on B7. so we didn't install the knock sensor and WINALDL is reporting a ton of knock and fouling the timing advance. We installed an MSD older HEI and used the tach lead to fire the injectors and it ran great. We would love to have the advantages of closed loop operation. The 402 is completely stock accept for headers by the way. It has the small cap separate coil dizzy. No esc or wiring for it in dizzy.

Thanx in advance for any help!

Lots of 7.4L did not come with knock sensor or ESC and any scantool will report loads of constant knock counts this way. But if you use the chip/bin that came with a no knock system it also has no knock retard set in chip/bin!

The reason your HEI distributor did so much better was the spark advance curve in it! From GM the same engine, without knock sensor, spark advance is set SO SAFE, it will never have spark knock. This was built that way so a one ton truck with wife hauling her four horse trailer... for example... could just drive and put her foot to floor and motor was always safe!

Another flaw in GM stock chip/bin for these trucks was fueling. Most are Async fuel and during Async the BLM Learn is disabled, even in Closed Loop. Open Loop idle is also the norm so when at idle it was smooth and not Stoich AFR of 14.7 to 1 AFR, this helped them cool down at idle. Also at idle and WOT the Air Pump is on to help heat up the CAT for emissions.

Emmissions was not really an issue on BB, 7.4L engines back then as the standards were different then passenger cars and light trucks. Remember that same engine with TBI wa the same engine HD trucks had with a carb. Hence the adapter for TBI.

What I do when tuning these early 7.4L TBI engines is add the correct knock sensor and ESC, start the tune with a 5.7L Sync fueling bin and change spark advance table to one that is simalar to what a HEI distributor with mechanical and vacuum advance would be. POOF! A good running BB TBI like it should have been from factory! :rockon:

Can you post the .bin file your using?

I'm thinking most of the problems your having can be fixed in the .bin file.:thumbsup:

How are you set up to burn chips?


I abandoned WinALDL years ago, as did most of us here. For the money, TunerPro RT is SUPERIOR to WinALDL!!!!

dave w

Thanks for the info Mark. What confused me was that there was a knock sensor on the 454 and factory wiring to the third (now removed) plug at the ecm. I would love to experiment with tuning and burning but I don't have the equipment for that and this is my friends truck so I think I will let someone else have that job. If it were up to me I would have got something like a Megasquirt ecm. Dave, I have no clue which bin is in there. It is the stock ecm from the 1991 GMC 3500. This is what I do have: ecm = 7747 other numbers on ecm are 86ANTY K383480987 and the software reported the PROMID = 8221 hope that helps and if you guys do any tuning by mail let me know. This is not a max effort deal but rather looking for something like Mark described. Lean the idle a bit, I think the injectors are a little large for the 402 in open loop. Advance etc...

Thanx again!

ANTY is the bin.

It's a 1987 7.4L Auto bin.

3 plugs at the ECM? The '7747 only ever had 2 plugs. Do you have any pictures of the harness? What was the colour of the 3rd connector?

The '7747 uses an external EST module that is usually mounted to the firewall, or a bracket on the intake of the engine ('90 to '94 Full Size).

The Delco ECM is a great ECM, and in many ways better than a MegaSquirt ECM, though the '7747 is one of those ECMs that while it has been thoroughly hacked and used in many non-stock applications and set-ups, it is an early ECM and therefore leaves a lot to be desired. Many people that really want to tune the TBI set-up will swap to the later TBI PCM found in the '93 to '95 automatic trucks. There are a few different service numbers that were used, most common is the 16197427 PCM that has been documented quite well, and even patches out there to add E-fan control and MAF control. There are other service numbers that are basically the same PCM. The other semi-common one that comes up is the 16147060, that was used on many 454/4L80E trucks. I have tuned a couple of these in friends vehicles but they are not as well documented and a little harder to find than the '7427, if a replacement is ever needed, though it is unlikely.

Pretty sure the third plug that was in the harness went to the cluster\fuse box. The harness had no wire just an empty hole for the esc at ECM but did under the hood and that's what caused the confusion, the bracket that held the MAP is where the esc should have been but it was never there. There was also no plug anywhere in the harness for the ESC. Strange I know.

ANTY is a factory no knock system. Error 43 is unchecked and knock retard set to 0.

Look at the picture of the spark advance table. See the dot in the hole? That's where your driving most of the time...

http://www.gearhead-efi.com/Fuel-Inj...1&d=1379211594

Downloaded the rt software were going to try to tune it ourselves. Should be an adventure...lol Will be a couple of weeks to get a burner and the time to get back to it. Thanks for all the info everyone.

Ok ordered the flash conversion adapters and a burn2. This may be a noob question but were all the 747 ecu's identical? We have one from a 4.3 s10 and he would be more comfortable modifying that one.

Yes, only the cal EPROM and the NETRES (the other small removable chip on the daughter board) were different between applications.

Cool so we also have a back up :)

Thanx again!

Update: The burn2 showed up in record time from Moates, read the chip (after the addressing problem was solved via forum search) then it was easy stuff. Still waiting for the other parts in the snail mail (different company). Then I will get on to tuning or de-tuning then starting over.... :) Learning is fun stuff until parts hit the wall...lol

Well, the G2 install went well. Will start tinkering in the morning. Truck fired up first try. Big relief.

Ok, humbling experience it was! Had some success and some failure. Went too rich moved a stumble around and had to punt...lol Back at it in a couple of weeks when he gets the truck legal to drive and we can collect some useable data. It was fun though!

As if I needed this to get any more complicated... my buddy calls and wants to change the cam and intake uggg so I picked a cam and would very much appreciate any feedback and or experience with this one please. Any suggestions for an intake?

COMP Cams 249-11-302-4

LSA = 112
Exhaust Valve Lift (in.) 0.503
Intake Valve Lift (in.) 0.503
RPM Range 1000-7000
Int. Duration @ .050 in. (Deg) 212
Exh. Duration @ .050 in. (Deg)218
Int. Duration Advertised (Deg) 260
Exh. Duration Advertised (Deg)268
Engine Make/Size:Chevy Big Block (396-454)
Lifter Type:Hydraulic Flat Tappet
Usage:Street/Performance

I don't know exactly what cam is in my Dad's 454, but I can tell you (and Mark can as well lol) that with a "computer friendly" (lie) cam, Edelbrock intake, and headers, it was about 20% lean. Took a fair bit of work adjusting the fuel pressure to get fueling back in line, and with as much work as I had to do to try and get fuel pressure up, any more fuel needed and it would have required a change of the fuel pumps.

On the potentially helpful side, I learned that if you have the OE 454 fuel pressure regulator that has the "window" where you can see the spring (some you can't), you can shim the spring until it's a hair less than at coil bind, and you will get as much pressure from the pump as it will deliver.

The thought I have is that this engine is 52 cubic inches smaller (402) so maybe the fuel would be in the adjustable range. The last tune I had in it was bad rich pulled hard but left a cloud...lol What you said makes sense that part combo should have netted at least a 20% power increase or more. The intake needs to be changed due to an obstruction on the stock intake the EGR stuff I think but it's wide and flat and will collect puddles of fuel. So his thinking is that he wanted a cam and if he has to pull the intake off he's not doing it twice and I can't argue that point. That was the smallest cam at least in the group I looked at. my question is really about how friendly that particular cam is to the computer. Because power is nice but drivability ultimately is going to be more important.

That should not be a hard cam to tune. I tuned more cam on a tighter lsa in a 305 running a 7747. Should idle all day at 650 rpm.

I think the rpm range of that cam is a lie or typo however I just don't see it making much power over 5,000-5,500 rpm. Definately not enough duration for 7K.

5000 is all it will ever see and hardly even that, didn't think it would do much more with the 650 or so cfm. It's a lot more cam than stock so it should give him a little more pop. I just wasn't sure how the system would handle it.

Thanx for the input!