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Hello everyone!
I just recently completed a 305 TBI to remanufactured 355 TBI engine swap in my 88 g20 van with a TH400 trans and have been having some issues with inconsistent power production and a possible lean condition. It was stumbling pretty bad during hard acceleration the other day, which seems to have been cleared up by redoing the base timing and idle (although that particular day was much colder than it's been lately so I'll have to try again on a cold day and report back.)
I guess I'm just looking for some clarification and maybe a couple leads/things to check into. Thanks!
- Fuel pressure was measured at a consistent 12psi at idle, cruising, and WOT during a 20 minute test drive. Unfortunately the fuel pressure gauge must have broken after that first drive because when I went for another drive later that day, the gauge was stuck at 0psi the entire time. For now lets assume fuel pressure is ok.
- No drips from the injectors, spray pattern appears to be a nice cone shape and consistent, but I'm no expert so take that assessment with a grain of salt.
- BLM was pinned at 172 for most of the drive, with periodic dips to the 135-150 range under ~75%+ throttle.
- INT was jumping between 125-150 for the first 3/4 of the drive and then it seemed to stabilize around the 125-135 range for the tail end of the drive.
- Power felt pretty solid and throttle was responsive during the entire drive (again, it was 45-50 degrees that day, I'm curious to try it again when its 35)
- I'm still in the process of investigating vacuum leaks but almost all the lines have been confirmed good and vacuum at idle is about 20"
- Base timing is 0* BTDC (potentially a hair advanced, maybe 1*) and base idle in drive goes between 750-500RPM depending on how long it sits at idle.
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I guess I'm still a little cloudy on how the BLM could be so high while the INT is just a little high/kind of close to 128. Do I just need to drive it more so the computer can get used to it's new home? It's a remanufactured engine that I've put 100 miles on so far and I'm just paranoid about glazing the cylinders if it is indeed running as lean as the BLM would suggest.
I have noticed an occasional (emphasis on occasional) loud hissing/sucking coming from the passenger side of the engine that gets louder upon acceleration, usually when it's still warming up. I'm assuming it's the EGR valve but I really can't tell. I plugged the line going to the EGR solenoid and it didn't seem to change anything but again, it's pretty intermittent.
From the junkyard:
intake manifold that I thoroughly cleaned
350 injectors out of an 89 pickup that I replaced the screens/o-rings on
ESC module,EGR solenoid, and MAP sensor from the 89 pickup
1227747 and PROM/MEMCAL from a 90 g20 with a 700r4 trans
From the old 305:
Throttle body (cleaned, spring and gaskets replaced)
O2 Sensor (about a year old)
EGR valve (old but feels like it moves fine)
Thermostat (about a year old)
Alternator (about a year old)
1. The computer is in "rich command", but that doesn't mean the engine is actually running lean. It means the computer is compensating for (correcting) a lean condition that may or may not exist any more. If the Integrator (short-term fuel trim) goes to rich-command, the Block Learn (long-term fuel trim) goes richer. If the Integrator is still rich-command, the Block Learn continues to go rich-command. Eventually the Integrator becomes happy, leaving the Block Learn semi-permanently rich-command. So, yes, the short-term trim doesn't need to go rich-command because the long-term fuel trim has adequately taken care of the leanness. The "sucking" sound indicates a vacuum leak; but they typically don't come-and-go intermittently. Worth investigating.
2. WHICH "new" knock sensor did you install? At least in the light-truck world, 305 uses a different knock sensor than 350. Was it properly torqued to spec with sealer on the threads? Over- or under-tightening will change it's sensitivity.
3. Fuel pressure is within "spec"; assuming your gauge is accurate. Have you verified the gauge actually shows 12 psi when connected to a verified 12-psi source? Pressure gauges "can" be wildly incorrect; and things have not gotten better since the Communist Chinese have flooded the market. Additionally, it's not unheard-of for a vehicle to need more fuel pressure than "spec" calls for--although that's typically needed when the engine is more-modified than yours.
4. False "lean" signal to the computer often happens when A) the O2 sensor is faulty; or B) when the engine is misfiring. Therefore...
5. ...Connect a scan tool, verify ALL the sensors (especially coolant temperature, MAP, knock sensor, and O2.) Assure that the spark advance is working, and not getting excessive spark-retard from a goofy knock sensor. Have you assured that the EGR actually works, and is the correct style for the engine? (My '88 K1500 5.7L has a "negative backpressure" EGR valve, I assume your 5.7L needs that also.) Your variable idle speed--500 to 750--indicates potential problems with the IAC; or may be related to the vacuum leak(s).
6. No idea how a chip for a 700 trans will interact with a vehicle having a 400 trans. There'll be no torque converter clutch to control, for example.
Last edited by Schurkey; Nov 28, 2022 at 11:49 AM.
I installed a 350 knock sensor, but I don't think I followed any sort of torque spec. I'll take another look at that one, but according to the scan tool, it's only detecting knocks immediately as it starts up and at the last second when it's winding down after powering off.
I'm going to try and hook up a different pressure gauge and see what happens.
As far as sensor monitoring goes, coolant temp sensor is reading true, O2 is dancing all over between .1mv and .9mv when in closed loop. I'm pretty sure the map sensor is reading ok, It seems to follow RPM pretty closely when graphed together.
The EGR may or may not be correct for the engine. I'll have to dig into that more.
It's not like the idle speed is bouncing between 750 and 500 over and over, but if you're driving and come to a stop light, idle will go to about 750 and then over the course of 30-45 seconds will kind of settle down to closer to 500, or at least it was that day. The IAC is brand new and I did the reteach procedure.
As far as the trans mismatch, Brian from Tuned Performance said that a 700r4 calibration should work fine with a TH400. The only issue I had was with the kickdown, as the 400 is electronically controlled (I believe by the ECM in this case) so I disconnected it and am planning on installing an older style kickdown wire attached to the gas pedal or something.
It seems like I should probably get several data logs over the course of a week or so, so that these assumptions can be based on a larger sample size than just one or two drives.
Potentially that rich-command could take some time to settle out, as I am understanding it?
I installed a 350 knock sensor, but I don't think I followed any sort of torque spec. I'll take another look at that one, but according to the scan tool, it's only detecting knocks immediately as it starts up and at the last second when it's winding down after powering off.
I'm going to try and hook up a different pressure gauge and see what happens.
As far as sensor monitoring goes, coolant temp sensor is reading true, O2 is dancing all over between .1mv and .9mv when in closed loop. I'm pretty sure the map sensor is reading ok, It seems to follow RPM pretty closely when graphed together.
The EGR may or may not be correct for the engine. I'll have to dig into that more.
It's not like the idle speed is bouncing between 750 and 500 over and over, but if you're driving and come to a stop light, idle will go to about 750 and then over the course of 30-45 seconds will kind of settle down to closer to 500, or at least it was that day. The IAC is brand new and I did the reteach procedure.
As far as the trans mismatch, Brian from Tuned Performance said that a 700r4 calibration should work fine with a TH400. The only issue I had was with the kickdown, as the 400 is electronically controlled (I believe by the ECM in this case) so I disconnected it and am planning on installing an older style kickdown wire attached to the gas pedal or something.
Why would you start with the 700r4 calibration? The 350 calibration can be copied into the 305/TH400 calibration. The TCC output could also be reconfigured on the 700R4 file to run the TH400. Both are simple.
12 PSI is a bit low. My all stock TBI system that I pulled from a 92 G20 had 14 psi and I have found 13-14 psi to run much better on even stock engines.
Have you verified the part number on the injectors? My brother had a 305 Firebird once that somebody had put 2.8L injectors in and then sold because it ran badly.
Why would you start with the 700r4 calibration? The 350 calibration can be copied into the 305/TH400 calibration. The TCC output could also be reconfigured on the 700R4 file to run the TH400. Both are simple.
12 PSI is a bit low. My all stock TBI system that I pulled from a 92 G20 had 14 psi and I have found 13-14 psi to run much better on even stock engines.
Have you verified the part number on the injectors? My brother had a 305 Firebird once that somebody had put 2.8L injectors in and then sold because it ran badly.
The 350/700R4 chip is all I could find at the junkyard. Seems like the 350/TH400 combo isn't nearly as common and I haven't been able to find one. I'm not making my own chips and have no plans to do so at this point.
As far as the fuel pressure goes, the plan right now is to replace the fuel pump next spring anyway. Probably do the adjustable fuel pressure regulator as well.
An no I have not confirmed the injector part number, I will definitely do that, good idea.
Why would you start with the 700r4 calibration? The 350 calibration can be copied into the 305/TH400 calibration. The TCC output could also be reconfigured on the 700R4 file to run the TH400. Both are simple.
12 PSI is a bit low. My all stock TBI system that I pulled from a 92 G20 had 14 psi and I have found 13-14 psi to run much better on even stock engines.
Have you verified the part number on the injectors? My brother had a 305 Firebird once that somebody had put 2.8L injectors in and then sold because it ran badly.
Well, double checking part numbers seems like a no brainer but apparently I have no brain.
Pulled the throttle body this weekend to put an adjustable fuel pressure regulator on it and I googled the injectors part number and lo and behold, they were for a 4.3L
They came out of a 5.7L pickup at the junkyard. Oops!
Ordered some refurbished 5.7L injectors from CFM tech and I'm pretty eager to see if that was the problem all along
Well, double checking part numbers seems like a no brainer but apparently I have no brain.
Pulled the throttle body this weekend to put an adjustable fuel pressure regulator on it and I googled the injectors part number and lo and behold, they were for a 4.3L
They came out of a 5.7L pickup at the junkyard. Oops!
Ordered some refurbished 5.7L injectors from CFM tech and I'm pretty eager to see if that was the problem all along
46 lb/hr vs 61 lb/hr should make a big difference in fueling.
BLM with the undersized injectors BLM with the correct, 5.7 injectors
Posting this for posterity.
Comparison between the 4.3L and 5.7L injectors.
Definitely a no brainer, but interesting to see nonetheless.
Now I'm wondering if it's running too rich. I got these injectors from CFM Tech and they were supposedly cleaned and flow matched, but the pods have a different look than the other injectors I have. No GM part number stamped on top, slightly different color metal, different looking tack welds on the bottom of the pods.
Last edited by DrPockets; Mar 18, 2023 at 10:09 AM.
Ever since I put in a new radiator, the engine temp stays cooler than before, around 180-185. Could it just be a little colder than the ECM wants it to be and as a result, richer? Should I put a different thermostat in there?
Eventually going to get rid of the clutch fan and switch to electric fans, so I could maybe have some more control over temperature then.
Late to the party as always...I tried poking around in tunerpro to see what influence the CTS would have. Unless I was using the wrong search terms, the only thing I found was mostly open loop items in which as you stated runs richer. However I'm sure you're in closed loop most of the time and the ECM will be targeting stoic 14.7 if that's what you have programmed into the tune. The only closed loop vs temp that I could find was a AE CTS multiplier. At 180dF, it adds a little more fuel but to me not a whole lot. AE is influenced by MAP changes and TPS changes which is your right foot. With a steady foot, you might not see any additional fuel from AE.
Idk what you aspire to do. Stock build, it'll behave and run off the stock tune? Your last datalog looks like you could benefit from a little tweaking of the VE tables. I actually prefer looking at the BLMs with the attached view...
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I guess I'm still a little cloudy on how the BLM could be so high while the INT is just a little high/kind of close to 128. Do I just need to drive it more so the computer can get used to it's new home?
To answer this specific question. No, the computer (ECM '7747) does not need to get used to it's new home. It is an antiquated thing and as no self learning capability. Only one engine cycle from cold to warm is enough. The BLMs (Block Learning Mode) are constantly updated, in closed loop, and only the last data is kept in memory (not an average) when engine is shut off. The BLMs represent small to larger areas of engine load and frankly are inaccurate. They are divided into 15 cells (blocks). BLMs are constantly updated as the engine travels through the cell areas.
The BLMs are updated based on the INT. The INT reacts instantly to lean/rich condition and adjust. BLM will be adjusted depending on what the INT is doing and at what rate. So a high BLM with INT around 128, means that the BLM has been updated using INT. The INT does not need to adjust anymore as the BLM is doing its job. Prior to your BLM going high, the INT must have been tapped out on the high side. Only a data log would confirm that. Also, the BLMs can vary as the cell# changes.
Formula is like this: INJ PW * BLM/128 *INT/128.
INT reacts based on O2 sensor voltage windows in closed loop. The windows are programmed to achieve stoichiometric combustion. The O2 sensor only monitors 1 bank on OEM set-up and needs to be hot enough to function properly. O2 sensor can be fooled and malfunctioning which would throw the BLM/INT off.
Hope this helps
Last edited by SbFormula; Mar 20, 2023 at 10:13 AM.
My fuel pressure is steady at 12psi currently, maybe it's a hair too high for the current tune?
Spec is 9--13. 12 psi is fine. That does, of course, assume that your pressure gauge is accurate. (Almost nobody bothers to test/verify their test equipment.)
It's a stock build and will most likely stay that way, but I am inching closer and closer to trying to do a little tuning myself just for fun.
My fuel pressure is steady at 12psi currently, maybe it's a hair too high for the current tune?
I would not worry about the BLM values you are seeing. I shoot for ~124ish anyway when I tune a TBI. They change at whim with the weather and intake air temperature as well as intake manifold temperature. Better to stay on the rich side and let the ECM pull a little fuel than be on the lean side.
I would not worry about the BLM values you are seeing. I shoot for ~124ish anyway when I tune a TBI. They change at whim with the weather and intake air temperature as well as intake manifold temperature. Better to stay on the rich side and let the ECM pull a little fuel than be on the lean side.
Awesome, thanks for the input.
By the way, as a fellow van guy combing through forums your posts have been unbelievably helpful and illuminating. Thank you!
