87 blazer tbi - no power

[bowtie5]

I have an 87 tbi 305 blazer. It seems as if the computer isn't controlling the timing correctly. I have replaced the EGR valve, the throttle body and injectors, plugs, wires, cap & rotor, and even checked the timing chain under the cover to see if it jumped. If I set the timing following the underhood sticker at zero degress with the timing plug at the firewall disconnected, I get a very very sluggish ride. I thought the computer would advance the timing to the best performance without any knock, but it doesn't change. If it put it at 10 advanced or so and plug the connector back up it will stay there also. the computer won't retard it back to zero, as i've been told. To get decent power I have quite a bit of spark knock. any ideas??

thanks

[tvblazer78]

You my have a bad module, try that. It should only be $20.00 or $30.00 for one.

Originally Posted by bowtie5

I have an 87 tbi 305 blazer. It seems as if the computer isn't controlling the timing correctly. I have replaced the EGR valve, the throttle body and injectors, plugs, wires, cap & rotor, and even checked the timing chain under the cover to see if it jumped. If I set the timing following the underhood sticker at zero degress with the timing plug at the firewall disconnected, I get a very very sluggish ride. I thought the computer would advance the timing to the best performance without any knock, but it doesn't change. If it put it at 10 advanced or so and plug the connector back up it will stay there also. the computer won't retard it back to zero, as i've been told. To get decent power I have quite a bit of spark knock. any ideas??

thanks

[bowtie5]

Is that the ignition module? in the distributor?

[tvblazer78]

Yep, I had one on a 89 chevy do about the same thing. So I put a new ignition madule on and it did the trick. So that mybe it.

Originally Posted by bowtie5

Is that the ignition module? in the distributor?

[ChevyTech]

Have you checked to see if the ECM has stored any trouble codes?

[bowtie5]

I don't have any trouble codes.

[ChevyTech]

Even at idle there should be timing advance.

If the ECM is working correctly, unplugging the wire to set the timing should have set a trouble code, and the pinging (spark knock) should set a trouble code.

The distributor module could be the cause, but I would expect an ECM trouble code because of the pinging.

I believe a 1987 has a spark knock module that is separate from the ECM. This module could be the problem.

Are you running a cold thermostat that is preventing a self test of the electronic spark control system?

I just noticed it is your first thread on the message board – WELCOME.

[bowtie5]

Thanks for the welcome

I am running a stock thermostat, a 195 I believe.

When i unplugged the est connector it did set a trouble code. when i reconnect it after setting the timing it does go away. when the spark knock was severe it did not set a code. I tried checking the knock sensor a few days ago but the connection looked fine. I forgot to plug it back up and during my short drive the amount of knocking was unbelievable. still didn't throw a code though. Is there a test for the module or just replacing it?

thanks

[ChevyTech]

I am running a stock thermostat, a 195 I believe.

Good

Is there a test for the module or just replacing it?

There are tests, but I found have them to not be very reliable at getting to the problem.

I forgot to plug it back up and during my short drive the amount of knocking was unbelievable.

It sounds like the electronic spark timing is working it the spark knock was much worse with the knock sensor unplugged.

Do you think the engine could have a build up of deposits in the combustion chambers? Heavy deposits can cause spark knock - “pinging”. Pinging will be detected by the knock sensor and cause the timing to retard causing a lack of power. You may want to try running some “Sea Foam” through the engine.

I have used SEA FOAM several times with great success. I have also used GMs TOP ENGINE CLEANER at the dealership many times with good results. The GM Top Engine Cleaner smells just like SEA FOAM to me and I have wondered if they are the exact same product. I have never had any problems from using these products, but if an engine has very heavy carbon, one treatment may not get it all out. My biggest concern is that it is not poured in so fast that it liquid locks the engine.

Use the instructions on the can – “When added thru injection or carb”.

[bowtie5]

I have browsed past posts up to becoming a member just recently and came across your mentions of "sea foam". I have run it through the truck. It sure does idle smoother and quieter, but no noticable increase in power. I just checked the cat converter to see if it was blocked up, but I get good pressure when I block off the tailpipe. Reading the "vague" haynes manual I was reading about each sensor to see what to check next, is testing the water temp sensor and ignition coil possible as a timing/ power problem?

[ChevyTech]

I will not try to discourage you from testing anything you would like to test. Helping people on a message board is helping by what the owner’s assessment and judgment makes them think is taking place. I can often get in a vehicle, start the engine, put it in gear, and in less then a minute have a real good idea of what is wrong. On a message board it is much easier to go in the wrong direction with the diagnosis.

I do not doubt there could be a spark timing problem. The early TBI trucks had the spark control active all the time. The later model systems do not use the knock sensor signal to control timing at all times, such as at idle in park.

The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging. Something loose rattling on or in the engine. Lifters, cam, rod, crank and piston noises all have the potential for causing false knock signals to be produced.

Loose torque converter bolts can cause a false knock signal to be detected.

Wiring harness location can also cause spark control problems.
Here is a quote from an engine rebuilding article I read;
The main wiring harness runs along the right valve cover. It has a "dip" in it that is supposed to go under the heater hose. If the installer turns the harness upside down so the "dip" goes over the heater hose, the harness is close to the plug wires and it will induce a small current into the harness...


My best guess at this point is you seem to have to much pinging to easily.
Thing that could cause this are:
High compression (carbon deposits)
In correct timing
Engine to hot
Poor quality fuel
Lean mixture

Another possibility is a MAP sensor not bad enough to set a code but not good. Check the condition of the vacuum hose to the MAP sensor also.

[bowtie5]

I have checked the engine for loose components as well as tightened the torque converter bolts.

My wiring harness is over the right valve cover.

Is there a test for the MAP sensor? I have good vacuum, no leaks there.

I adjusted the timing to about 7 or 8 degrees advanced and locked down the distributor. I filled up with premium. This is just before any pinging occurs, and it stays at that timing. This gives me the best performance i've had so far. I don't understand it, but it seems to work. I think I may just have to live with this "fix", rather than replace part after part.

[ChevyTech]

I am out of town without access to all my information on my computer. When I get home next week I can send you test information for the MAP sensor.

[ChevyTech]

If you want to do testing before I get home to my information here is what I can tell you from the top of my head.

Of the three wires going to the MAP sensor:
One is approximately 5 volts – it is the “reference voltage”
One is a ground.
One is the voltage that the ECM will read.

I always suggest using jumpers so the wires do not have to be pierced to do the testing.

When the vacuum is low the voltage should be high.

When the vacuum is high the voltage should be low.

With the engine not running I believe the voltage should be around 4.5 volts.

At idle I think the specification is around 1.25 volts with a fairly large tolerance around it.

[ChevyTech]

Here is general information on MAP sensors for TBI trucks. If it does not look right for your truck, say so, and I will look it up. I am a bit busy as I just return home from out of town.

A normal Map sensor reading on the wire that connects to terminal “B” (possibly Lt green) would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

NOTE: if it runs poorly it could have much lower vacuum at idle which would cause a higher voltage reading.

Terminal “A” (possibly Gray) is voltage supply to the Map sensor. (Should be close to 5 volts)

Terminal “C” (possibly purple) is the ground for the Map sensor. (Should have no voltage)