'88 OMC Cobra 4cyl 3.0 - Cyl mystery

[mbobo]

Just obtained this '88 Sunbird which I witnessed running last summer from a distance. It was left out with no winterization this past winter and now it's my project. Previous owner reported overheating - I replaced all water hoses and the non-functioning thermostat. Engine will not start - sort of.

It turns over fine but will not start, just coughs. Pulled the #1 plug to check for a spark and when we turned it over the engine started and ran on three cylinders. Very rough as you might expect, but it continued to idle. Put the plug back in and no start. Changed the plugs and wires, still no start. Checked for a clear exhaust path, still no start.

My suspicion is a stuck valve, but I'm not sure of my analysis. Any help would be greatly appreciated!

Thanks,

Michael

 

[Windykid]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

Pull the valve cover and check for a broken spring. But it is an unusual problem that you pull a plug and it runs. Is the motor cranking fast enough with the plug in?

 

[Coors]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

Did you re-check the firing order? not where the wires were when you got it, but by the manual?

 

[Limited-Time]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

First off check for water in the oil, if the block froze you may be trying to start a junk engine. Second a quick compression check will indicate the general condition of the valves, rings and head gaskets. An overheat will often times take out a head gasket. If the compression looks good make sure you plug wires are in the correct firing sequence.

 

[mbobo]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

No indication of water in the oil.

It does seem to turn a bit slow when cranking, what's the solution for a faster spin rate on the starter?

The original 4 plug wires were all custom lengths, so I know from that end that I replaced them correctly, I'll check the distributor cap against the manual to make sure the wires are originating correctly.

I don't own a compression tool, but I do think I can borrow one from a neighbor.

Just from an easy perspective, I'll do those three before pulling the valve cover and dealing with finding a new gasket.

Thanks for the quick responses! I'll report back with success or any temporary setbacks.......

Michael

 

[Jsiples]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

I am not a pro like the rest but I figured I would say regarding the overheating that the impeller would need to be changed asap. I have read many posts on that and it is a cheap item that will destroy things if it is not replaced. Also, I have made the mistake of once forgetting to tighten the petcock on the block also which made starting difficult, just some things to look at while your working to help if not done already.

 

[mbobo]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

Just a quick update on my project.

After putting the pieces back together the engine fired and seemed to run well. We made some carb adjustments, but it ran very rough below 1000 RPM. Pulled the plug wires one at a time and each had a dramatic effect except for #4, virtually no impact. Check for a spark and that's confirmed.

My next step is a compression test - the saga continues......

 

[mbobo]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

I just ran a compression test and would love some input as to what the numbers might mean, the results as follows:

1 - 141
2 - 135
3 - 120
4 - 90

As it falls off rather dramtically close to the back of the engine, could I have a blown head gasket toward that end?

Thanks,

Michaell

 

[bsh21wash]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

oh...not good... anything under 100 ur just burning gas and with the prices right now...ouch

 

[Coors]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

Do a leak down test before you pull the head.

 

[Don S]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

When you pulled the spark plugs out, did you see any rust on #4, or perhaps it looked like new???? and like it hadn't fired?
Before you go to the trouble of an actual leakdown test, try squirting about a tablespoon full of oil into #4 and see if the compression comes up or not and checking out this service bulletin.

Service Bulletin 97-25

 

[mbobo]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

Wet compression test results added to the data below:

Cyl Dry Wet
1 141 173
2 135 150
3 120 175
4 90 115


All 4 plugs look the same, dry and black, which I believe is an indication of carbon? I don?t have the tools for the leak down test described in the service bulletin, but perhaps I can find what I need this weekend.

In the meantime I need to get the boat off the owner?s trailer, will it do any further damage to run the boat for about ten minutes to launch and move it to a covered slip to continue work?

By the way, you guys are awesome for helping out!

Michael

 

[Don S]

Re: '88 OMC Cobra 4cyl 3.0 - Cyl mystery

You don't need a leakdown test at this point. You have bad rings, plus a valve problem on #4.
Basically, and I know you don't want to hear this, you need an overhaul or a new long block.
Won't hurt to run it to your dock. But your going to need it out of the water to pull the engine.