Hey guys I have a timing issue.

Hey, I have a 1988 k5 blazer with tbi 350/700r4/np208. I have this strange issue where when i start driving, the timing advances then immediately starts retarding, causing the truck to be very sluggish. This is accompanied by a awful idle when warming up and sometimes code 45 (running rich). I have replaced coil with accel super coil, e3 plugs, brand new ac delco knock sensor, carquest plug wires, duralast dristributor (with ac delco original icm and accel cap+rotor) ive swapped the knock sensor but it still retards, could it possibly be an issue with the esc above the passenger valve cover??? is the icm able to retard timing??? any help is greatly appreciated!!!

Also, I forgot to mention, sometimes it starts right up then immediately dies, then i crank it and it starts, stumbling for a few seconds then smooths out.

How are you verifing that the timing is being retarded? Have you run a fuel pressure test? When was the fuel filter last changed? How old is your O2 sensor? Is it changing voltage? Did you hook a scanner to the truck and look at live data to see what is happening when you start having driveability issues?

evilunclegrimace said:

How are you verifing that the timing is being retarded? Have you run a fuel pressure test? When was the fuel filter last changed? How old is your O2 sensor? Is it changing voltage? Did you hook a scanner to the truck and look at live data to see what is happening when you start having driveability issues?

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I used a timing light and would rev it lightly and the timing would spike then start retarding and the engine would bog slightly, feel it while driving too out of a stop it revs up perfect then loses all its guts. Fuel pressure is at 12-13 psi perfect. Fuel filter is around 2-3 weeks old. 02 sensor i ditched a bosch for a delphi when the issue started probably a couple weeks old. I dont have a scanner and cant really afford one due to tight budget for repairs, but i have been testing sensor with a multimeter as much as possible.

Nick88 said:

I used a timing light and would rev it lightly and the timing would spike then start retarding and the engine would bog slightly, feel it while driving too out of a stop it revs up perfect then loses all its guts. Fuel pressure is at 12-13 psi perfect. Fuel filter is around 2-3 weeks old. 02 sensor i ditched a bosch for a delphi when the issue started probably a couple weeks old. I dont have a scanner and cant really afford one due to tight budget for repairs, but i have been testing sensor with a multimeter as much as possible.

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With out live data you are at a stand still, swapping out any more parts at this time would be guessing. Check around for a shop that can read live data when you can.

evilunclegrimace said:

With out live data you are at a stand still, swapping out any more parts at this time would be guessing. Check around for a shop that can read live data when you can.

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I did test voltage and ohms on most sensors, some backblading for more live type data from them, I get that is a far from perfect way of doing, but i was more wondering if the icm can retard timing in case its possibly an issue with that or if it could be an issue with the esc? I know for a fact it is related to timing. I was just wondering about what could be the reason based off of what the different sensors/modules do.

Nick88 said:

ive swapped the knock sensor but it still retards, could it possibly be an issue with the esc above the passenger valve cover??? is the icm able to retard timing???

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The knock sensor may be telling the ESC there's knocks or vibrations going on, and the ESC will retard timing. Other things like rattling exhaust pipes, loose motor mounts, etc.

The Knock Sensor tells the ECM there's knocks. Then the ECM changes timing to the ICM so, yes the ICM retards timing.


Here's some FSM for you. They should help diagnose your issue(s).

One thing, stay away from all those gimmick components like e3 spark plugs, Accel stuff, etc. Stick with GM Original Equipment (if you can find them) ignition parts...

PlayingWithTBI said:

The knock sensor may be telling the ESC there's knocks or vibrations going on, and the ESC will retard timing. Other things like rattling exhaust pipes, loose motor mounts, etc.

The Knock Sensor tells the ECM there's knocks. Then the ECM changes timing to the ICM so, yes the ICM retards timing.

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Here's some FSM for you. They should help diagnose your issue(s).

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

One thing, stay away from all those gimmick components like e3 spark plugs, Accel stuff, etc. Stick with GM Original Equipment (if you can find them) ignition parts...

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Thanks for the help! Do you think a bad esc or esc connector could cause bad timing even if the knock sensor is right, I just drove it and the truck is like a rocket when you get in it a bit more and timing advances without retarding, seemingly most of the sluggishness snd timing retarding is in the low end (how you would drive normally around town compared to ripping it on an on ramp basically). Could a loud exhaust leak or loud exhaudt in general cause bad knock sensor counts/ could possibly over tightening knock sensor cause sensitivity issues (harbor freight torque wrench eeeek). Im just trying to rule out everything as the truck is so good when it runs right. I know the motor had minor vibration from worn motor mounts but not enough to cause anything in my opinion. Also yes I know to stay away from gimmicks but I figured an accel coil is better than a duralast no matter what and a little more heat cant hurt, also the e3s i got massively discounted it was probably 38 bucks for 8 before tax brand new. I figured a couple extra straps on the plug couldnt hurt.

Yes over tightening a knock sensor can cause issues. If I remember correctly the spec is 14 lb.ft. I will double check here in a minute.

evilunclegrimace said:

Yes over tightening a knock sensor can cause issues. If I remember correctly the spec is 14 lb.ft. I will double check here in a minute.

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i figured it was 14 but im not sure how accurate the harbofreight wrench is, 14 could be 18 for all i know with that thing