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1987 Chevy K5 Blazer, 5.7L engine 4-speed auto. Complaint – TBI Injectors won’t pulse fuel when cranking the engine, then after spraying either into the throttle body it will start and the TBI injectors spray fuel but the engine will surge from about 800 RPM to about 1800 RPM in a cyclic rhythm. I started by repairing the obvious issue of the injector wiring harness that was burned with exposed wires / issue still persisted. I then noticed one of the two injectors was not spraying fuel, it was just dribbling fuel as the other sprayed. I rebuilt the TBI unit and got it to operate properly (both injectors spraying fuel) / issue still persisted. I attached my ancient “brick” Snap-On scanner to the OBD I connector and was able to watch the available live information but all of the sensors were within specification and there were no trouble codes. I then taped into the fuel line to check the pressure. On initial prime the pump only showed about 3-4 PSI and while running it was only about 5-6 PSI. I replaced the in tank fuel pump, checked the initialization prime (11 PSI) and then the operating pressure (11-12 PSI) / issue still persisted. I then checked continuity of the wires going from the distributor to the ECM, no shorts or opens with perfect continuity. I then began to throw parts at it. I installed a new ECM / issue still persisted. I installed as new distributor / issue still persisted. I checked the reference wire from the distributor to the ECM by connecting a test light to battery positive and turned the ignition on (KOEO) and then touched the reference wire and the injectors would pulse. I think I’ve pretty much proven that each individual part of the system works on it’s own but I cant figure out why all of the parts wont work together.
Though you state all of the sensor's data shows normal: what is.the tps at? It may be reporting high voltage=full throttle and hence fuel cut during crank occurs. I've seen this more than once.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Were the sparkplugs filthy? I would think the egr and passage is filthy, possibly not sealing well, the iac is trying it's best, but there is unmetered air entering
I haven't checked the spark plugs but I will take them out and see what they look like. I also forgot to mention that I did attempt to do the IAC reset before I rebuilt the TBI unit. However I haven't attempted it since the rebuild. I'll give that a go tomorrow.
More follow on information I just got from the owner: The blazer ran fine and would start without any issues. He had a head gasket issue and "blew the engine up" so he replaced the motor with a stock crate motor from Jegs. He said that the TBI no start and pulsing issue started immediately after he finished the installation. He also told me that he replaced the IAC, IAC connector, Spark Module and Throttle Position Sensor but the issue dint resolve itself so the blazer found it's way to me.
I really appreciate the feedback and advice. I'll check on all of the suggestions given to me tomorrow and see what comes of it.
I apologize for the delay; my wife took leave for Thanksgiving so I took time off to be with her and our kids. Just got done checking all of the suggestions, here are the results:
IAC Reset - Complete
Throttle Position Sensor voltage - When the engine would surge it was at .94 to .98 volts at 800 RPM and 1.12 to 1.14 volts at 1250 RPM. However today is the first time it went from surging to a constant high idle intermittently. It would go up to 1500 RPM all on its own and the TPS voltage would be at 1.44 volts (and I never touched the throttle). I also noticed the IAC would go from 90 to 104 when it was surging and stay at 144 when it was high idling. I then noticed the O2 sensor would read Lean, the mV reading would be at .004 and it showed no cross counts. When at high idle it would read rich then lean, the mV reading was around.500 and the cross count would start to register. The whole time it said it was in a closed loop. Every time it would high idle, I'd let it stay there for about 2-3 minuets then I'd hit the throttle to WOT then it would back down and begin to surge again. It would surge for about 2 minutes and go back to the high idle.
Exhaust back pressure - None, it's got straight pipes from the manifolds out to the back.
Voltage Drop for the Grounds - Unfortunately I couldn't find an issue here.
Spark Plugs - They were clean, no carbon build up.
Would the PROM be causing any of this or should I just build a new harness between the distributor and the ECM? Thanks again for all of your assistance.
At this point i'm not sure surging or high idle is due to IAC issue or a vacuum leak. I suggest two tests.
1. While engine idling and not touching throttle, unplug IAC. Like if it were a carburetor, rpm should stay constant (surging should stop).
2. Still IAC unplugged,
very slowly block off IAC passage with one of your fingers. That should drop rpm to something about 700 rpm. Because you are reading TPS voltage above 0.8v, i'm guessing your new rpm should be high (if i remember well, factory setting is 0.45v).
On the other hand, there are 3 PID's that can help you to know whether or not you have an IAC calibration issue (target rpm, actual rpm and iac steps).
If target rpm is lower than actual rpm, iac steps tend to go down and viceversa.
Let's say engine is idling at 1500 rpm. Then ecu tries to reduce the amount of air by closing IAC passage. Then suppose iac steps goes to 0 and engine is still idling high. That means you have throttle blades too open or you have a vacuum leak.
At high idle the MAP is at 22 and .94 Volts. When the engine is surging the MAP is at 21 and .64V. My current local barometric pressure is 30.16 in (according to weather.com).
You are correct on my location being close to sea level, my shop is about 7.5 miles Northwest of the Crystal Coast on the Atlantic Ocean (Hubert, NC about halfway between Wilmington, NC and Morehead City, NC). My elevation is 30 feet above sea level. Based on my conversion chart 30.16 inHG (inches of mercury) is equal to 102.13 kilopascals (kPa). Based on your calculation I'm not certain of what numbers/units you're looking for?
Guafa is referring to the manifold pressure at (high) idle your eported. A sound engine at your altitude should produce an vacuum of around 18 to 21 ``Hg at idle. This would result in an ABSOLUTE pressure in the manifold of 10 to 12 ``HG. The number you`re giving is 22``Hg. This equals to to only 9``Hg which is a VERY low value.
Depending on the scantool you use it can be easy to mistake engine vacuum for absolute manifold pressure. IF the reported pids were swapped this would be represetative of a good breathing engine at idle condition....
Make sure you really are reading manifold vacuum as such and manifold pressure as manifold pressure. If in doubt about scan data just put a vacuum gauge on it.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
