Diagnose Speed Density ECM Controlled Engines - How does it work and what should I think about?

[@JohnCRV]

Good info Dan. Thank you We’re at this stage with the Z28. Runs great after fuel tank and pump plus new injectors…… but it needs a good adjustment.

[@bitsofwisdom460]

Sounds great! I hope you get the adjustments worked out.

[@AV131082]

Thanks for the info, great to keep it at hand!

[@bitsofwisdom460]

You're welcome!

[@mikebruegger8654]

Another great mass of helpful information! Thnx... 😊

[@bitsofwisdom460]

You're welcome!

[@wwclt]

Exactly....I have one of those and it reads zero. I happen to come across your video and thought I would reach out and see if you had any idea. I've NEVER seen anything like this B4 in my 40 years of wrenching . Check timing twice engine hasn't ran for 5 minutes so I'm sure the cam is ok

[@bitsofwisdom460]

Ok, just need to be sure of the big stuff. If you measured it, that should rule it out. These odd ones can really make you think. So, I go back to your comment that the engine ran well without the exhaust and after install it won't run. That suggests that either something else changed during the exhaust connections (such as wire or cable connection disturbed or shorted to ground) OR something totally unrelated died by coincidence at just the same time. I like to go down the logic paths - test and verify. Taking exhaust on and off is a pain, but you could still drop it enough to see if the engine will fire. Based on your back pressure measurement, I would expect that if you drop the exhaust, it won't run. If it runs, the question is what is changing at the same time. If it doesn't run, we go back to basics for a non-running engine - air - fuel - spark - cam - compression, timing and control system. Could it be a MAP sensor, yes it's possible. Could also be MAF, Cam Sensor, Crank Sensor - yes. You can try disconnecting MAP or MAF to see if you get any change. It would help a lot to get a data scan on it to see if there are any background or history codes and what the base parameters are. Sensor data on a dead engine and during cranking can be helpful. Cam and crank sensors should look normal during crank.

[@wwclt]

Ok.... FINALLY....it runs. I took your advice and hooked the cats back up and then I hit me . I only removed the upstream O2 's with the cats hooked up however when I took the exhaust completely off there were no O2 's hooked up and the truck would run , so I decided not to hook the down stream o2's with the cats hooked up and it fired up and runs so the problem was in the rear O2 sensors now what I don't no yet, I will investigate it more tomorrow.....thx for your help

[@bitsofwisdom460]

@@wwclt You're welcome. I'd be interested in what you find.

[@sorewitheagles]

Hi Dan, should I change a manifold air temperature sensor that measures a resistance of 1800 ohms with an air temperature 180 deg F (from a hair dryer and an oven thermometer directed in the throttle body)? GM shop manual, for 1989 "F" code 5.0L TPI 5-speed (original owner), shows that 1800 ohms should be caused by a temperature of only 100 deg F, on page 6E3-A-37. Would this large difference in temp confuse the ECM, causing the chuggle (GM's term for surging) to occur once the engine is running and warmed up? I am currently chasing down the cause of this surging with a multimeter. At 70 deg F, the sensor resistance is close to Gm spec. Thanks for any advice!

[@bitsofwisdom460]

If the sensor is that far off, I would replace it. The caution would be if you are confident that you're getting a good reading of the temperature so that the sensor is actually at the temperature you expect. From what you describe, it sounds like it should be. I don't know the specific logic that the ECM uses to adjust the fuel mixture for air temperature, but the engine only uses mass air flow, coolant and air temp, engine speed and throttle position as primary inputs. I expect that it makes adjustments to fuel mixture and ignition timing that would likely affect your issue.

[@sorewitheagles]

@@bitsofwisdom460 I removed the MAT sensor , cleaned it and tested with hot water and accurate thermometer, at various temperatures. Ohm results were quite close to the chart in the GM service manual. I ran the engine with a potentiometer connected in place of the MAT sensor and it had very little change in the idle quality, at any resistance value in GM's chart. I'll check a few more things, and then look into connecting a scan device as suggested in your other videos. Much thanks for your help, Dan!

[@6904gabriele]

Hi Dan I have a question for you about the charcoal evaporator canister(305 TBI): the other day I found a great ammount of fuel in it and in the vent line from the tank too,my check valves are all new,the Camaro manual says to replace the filter in the canister but mine is sealed at the bottom,no way to open it so my idea is to delete that canister because of safe you know,I don’t want my Camaro in flames for that problem.What do you think?Is it possible to delete that?Thank you.

[@bitsofwisdom460]

You're correct that you need to address this issue to eliminate the liquid fuel for safety. I can't advise to remove or eliminate the canister, but rather find a way to make it work correctly. I'm not expert in this area, but this is my best understanding: There are two basic reasons to have fuel in the canister and the lines. First is overfilling the fuel tank. It is intended to have an air bubble at the top where the vapor line connects, but a high fuel level (perhaps more than 3 clicks at the nozzle) and possibly combined with parking on a slope can cause fuel to be pushed up into the vapor line. Second, there can be cases where vapors collect and condense in the canister. The canister has an area where some liquid fuel can collect to be drawn out in normal operation later. If the purge function isn't working correctly and drawing out the vapors, fuel may continue to collect and overload the canister. You need to verify that the lines and solenoid to the intake manifold are providing vacuum to draw vapor from the canister. I haven't seen a GM canister in the '80s cars that wasn't round with a filter in the bottom - it may be possible, but I don't know. They normally take in air from the bottom. I have seen some with the filter missing, but there ought to be some openings for air intake. If it was mine, I would try to blow out the lines with compressed air to remove liquid fuel. I don't know the best way to drain the canister. I have seen some others who have tried to flow compressed air through it or found ways to open the canister and replace the carbon, but I have no experience to add there. Only a suggestion for further study.

[@6904gabriele]

@@bitsofwisdom460 Yes,sometimes I overfill my tank,it’s my defect you know….. Perhaps that’s the reason,my other '88 Firebird has the same completely closed canister,I can try to swap on the Camaro and see.I have to search when that solenoid is active.The vacuum lines are all free.

[@wwclt]

I failed to mention that I disconnected the entire exhaust however I left the O2 sensors plugged in and it starts right up, sounds like it is ready for Drag strip so I'm positive the engine is fine. You think it could possibly be a bad map sensor... Thx

[@bitsofwisdom460]

If the engine runs well without the exhaust system and sets no codes for bad sensors other than O2 sensor codes that will set because the exhaust is open. (Those would be lean codes and poor catalyst efficiency). Then I don't think you have an engine issue, I would take one more look at exhaust, even if parts are new. Exhaust back pressure should be almost zero at idle. If that small amount of back pressure is upsetting the engine or engine controls, it seems extremely odd. It could perhaps happen if a cam is off timing or a lobe is going flat, but you should have an OBD2 cam sensor that is monitoring position / timing. If MAP sensor reads correctly with open exhaust, I wouldn't expect it to be bad. I would get a tool like one of these: www.amazon.com/Tool-Aid-33600-Exhaust-Pressure/dp/B0002SQW9S www.amazon.com/dp/B09CCWDKR6/ and check pressure at the O2 sensor ports on each bank and on each side of the cats.

[@wwclt]

I have a 2013 Ram 5.7 and it want start with exhaust connected however when i disconnect them the truck runs well.. This happened after cam and lifters replacement. Replaced the cats still not start however as soon i as remove the cats it starts right up....any thoughts...thx

[@bitsofwisdom460]

My first thought is still exhaust. There shouldn't be much backpressure at idle. If the engine runs without exhaust I would first rule out the rest of the exhaust. I don't know the history, but if you had cam and lifter failure causing misfire that took out the cats, the bricks in the cats can melt / fracture and travel down to the muffler and obstruct the pipe. Sometimes mufflers can have internal failure. I would either disconnect the exhaust behind the cats and test or get an adapter to thread into one of the post cat o2 sensor ports and get a pressure reading. If it won't start, at least get a cranking pressure reading. GM had a spec back in my Camaro days that said pressure should not exceed 1.25 PSI at idle and 3 psi at 2000 RPM with the cats in the system.

[@NotmeGK123]

All be don’t know if mine has crack shaft sensor I’ll look thanks a million well dang it mine doesn’t have crank sensor back to chasing tail lol

[@bitsofwisdom460]

If it still has a distributor (and I think it does), it won't have a crank sensor. Crank sensors do the job of the pickup coil in the distributor on distributorless engines.

[@NotmeGK123]

@@bitsofwisdom460 ok thanks this is why I liked KZbin years ago can talk to people about stuff now days to big for its britches lol still small time stuff just hard to find anyways thanks again

[@NotmeGK123]

My 91 k1500 acts fine one minute shut it off start back in 15 minutes acts funny for minutes I’ve changed all the sensors rebuilt TBI. Idk make you chase tail lol think it’s ECM

[@bitsofwisdom460]

It would seem odd to be an ECM, unless you find a specific cause. I had one recently with a similar symptom that had a leaking fuel pressure regulator. Pressure was correct, but it would bleed fuel through the diaphragm into the vacuum hose when shut off and make the engine rich on restart.